mention use of underscore for private fields in manual

doc/src/manual/style-guide.md

@@ -141,6 +141,10 @@ uses (e.g. `a[i]::Int`) than to try to pack many alternatives into one type.
  words squashed together ([`isequal`](@ref), [`haskey`](@ref)). When necessary, use underscores
  as word separators. Underscores are also used to indicate a combination of concepts ([`remotecall_fetch`](@ref)
  as a more efficient implementation of `fetch(remotecall(...))`) or as modifiers.
+ * functions mutating at least one of their arguments end in `!`.
+ * use identifiers starting with `_` to
+ denote functions, macros or variables that should be considered private and not part of a package's
+ public API.
  * conciseness is valued, but avoid abbreviation ([`indexin`](@ref) rather than `indxin`) as
  it becomes difficult to remember whether and how particular words are abbreviated.
 

doc/src/manual/variables.md

@@ -141,5 +141,7 @@ conventions:
  * Functions that write to their arguments have names that end in `!`. These are sometimes called
  "mutating" or "in-place" functions because they are intended to produce changes in their arguments
  after the function is called, not just return a value.
+ * Names starting with an underscore denote functions, macros or variables that are only used internally
+ by a package and are not part of its public API.
 
 For more information about stylistic conventions, see the [Style Guide](@ref).