Document the semantics of annotation ordering

It's important to specify the way that annotations relate to the
characters of the underlying string and each other.

Along the way, it's also worth explaining the behaviour of the internal
functions _clear_annotations_in_region! and _insert_annotations!.

base/strings/annotated.jl

@@ -25,6 +25,17 @@ and a value (`Any`), paired together as a `Pair{Symbol, <:Any}`.
 Labels do not need to be unique, the same region can hold multiple annotations
 with the same label.
 
+Code written for `AnnotatedString`s in general should conserve the following
+properties:
+- Which characters an annotation is applied to
+- The order in which annotations are applied to each character
+
+Additional semantics may be introduced by specific uses of `AnnotatedString`s.
+
+A corollary of these rules is that adjacent, consecutively placed, annotations
+with identical labels and values are equivalent to a single annotation spanning
+the combined range.
+
 See also [`AnnotatedChar`](@ref), [`annotatedstring`](@ref),
 [`annotations`](@ref), and [`annotate!`](@ref).
 
@@ -317,6 +328,9 @@ end
 
 Annotate a `range` of `str` (or the entire string) with a labeled value (`label` => `value`).
 To remove existing `label` annotations, use a value of `nothing`.
+
+The order in which annotations are applied to `str` is semantically meaningful,
+as described in [`AnnotatedString`](@ref).
 """
 annotate!(s::AnnotatedString, range::UnitRange{Int}, @nospecialize(labelval::Pair{Symbol, <:Any})) =
  (_annotate!(s.annotations, range, labelval); s)
@@ -349,6 +363,9 @@ annotations that overlap with `position` will be returned.
 Annotations are provided together with the regions they apply to, in the form of
 a vector of region–annotation tuples.
 
+In accordance with the semantics documented in [`AnnotatedString`](@ref), the
+order of annotations returned matches the order in which they were applied.
+
 See also: `annotate!`.
 """
 annotations(s::AnnotatedString) = s.annotations
@@ -483,6 +500,15 @@ function write(dest::AnnotatedIOBuffer, src::AnnotatedIOBuffer)
  nb
 end
 
+"""
+ _clear_annotations_in_region!(annotations::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, span::UnitRange{Int})
+
+Erase the presence of `annotations` within a certain `span`.
+
+This operates by removing all elements of `annotations` that are entirely
+contained in `span`, truncating ranges that partially overlap, and splitting
+annotations that subsume `span` to just exist either side of `span`.
+"""
 function _clear_annotations_in_region!(annotations::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, span::UnitRange{Int})
  # Clear out any overlapping pre-existing annotations.
  filter!(((region, _),) -> first(region) < first(span) || last(region) > last(span), annotations)
@@ -508,14 +534,24 @@ function _clear_annotations_in_region!(annotations::Vector{Tuple{UnitRange{Int},
  annotations
 end
 
+"""
+ _insert_annotations!(io::AnnotatedIOBuffer, annotations::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, offset::Int = position(io))
+
+Register new `annotations` in `io`, applying an `offset` to their regions.
+
+The largely consists of simply shifting the regions of `annotations` by `offset`
+and pushing them onto `io`'s annotations. However, when it is possible to merge
+the new annotations with recent annotations in accordance with the semantics
+outlined in [`AnnotatedString`](@ref), we do so. More specifically, when there
+is a run of the most recent annotations that are also present as the first
+`annotations`, with the same value and adjacent regions, the new annotations are
+merged into the existing recent annotations by simply extending their range.
+
+This is implemented so that one can say write an `AnnotatedString` to an
+`AnnotatedIOBuffer` one character at a time without needlessly producing a
+new annotation for each character.
+"""
 function _insert_annotations!(io::AnnotatedIOBuffer, annotations::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, offset::Int = position(io))
- # The most basic (but correct) approach would be just to push
- # each of `annotations` to `io.annotations`, adjusting the region by
- # `offset`. However, there is a specific common case probably worth
- # optimising, which is when an existing styles are just extended.
- # To handle this efficiently and conservatively, we look to see if
- # there's a run at the end of `io.annotations` that matches annotations
- # at the start of `annotations`. If so, this run of annotations is merged.
  run = 0
  if !isempty(io.annotations) && last(first(last(io.annotations))) == offset
  for i in reverse(axes(annotations, 1))