Base: Describe heredocs in Shell's manpage

Base/usr/share/man/man5/Shell.md

@@ -44,6 +44,17 @@ Any sequence of _Double Quoted String Part_ tokens:
 * Evaluate expressions
 * Escaped sequences
 
+##### Heredocs
+Heredocs are made in two parts, the _initiator_ and the _contents_, the _initiator_ may be used in place of a string (i.e. wherever a string is allowed to be used), with the constraint that the _contents_ must follow the _sequence_ that the _initiator_ is used in.
+
+There are four different _initiators_:
+- `<<-token`: The _contents_ may contain interpolations, and are terminated with a line containing only whitespace and then _token_
+- `<<-'token'`: The _contents_ may _not_ contain interpolations, but otherwise is the same as `<<-token`
+- `<<~token`: Similar to `<<-token`, but the starting whitespace of the lines in the _contents_ is stripped, note that this happens after any and all expansions/interpolations are done.
+- `<<~'token'`: Dedents (i.e. strips the initial whitespace) like `<<~token`, and disallows interpolations like `<<-'token'`.
+
+Note that heredocs _must_ be listed in the same order as they are used after a sequence that has been terminated with a newline.
+
 ##### Variable Reference
 Any sequence of _Identifier_ characters, or a _Special Variable_ following a `$`.
 Variables may be followed by a _Slice_ (see [Slice](#Slice))
@@ -387,7 +398,9 @@ and_logical_sequence :: pipe_sequence '&' '&' and_logical_sequence
  | pipe_sequence
 
 terminator :: ';'
- | '\n'
+ | '\n' [?!heredoc_stack.is_empty] heredoc_entries
+
+heredoc_entries :: { .*? (heredoc_entry) '\n' } [each heredoc_entries]
 
 variable_decls :: identifier '=' expression (' '+ variable_decls)? ' '*
  | identifier '=' '(' pipe_sequence ')' (' '+ variable_decls)? ' '*
@@ -407,7 +420,7 @@ control_structure[c] :: for_expr
 continuation_control :: 'break'
  | 'continue'
 
-for_expr :: 'for' ws+ (('enum' ' '+ identifier)? identifier ' '+ 'in' ws*)? expression ws+ '{' [c] toplevel '}'
+for_expr :: 'for' ws+ (('index' ' '+ identifier ' '+)? identifier ' '+ 'in' ws*)? expression ws+ '{' [c] toplevel '}'
 
 loop_expr :: 'loop' ws* '{' [c] toplevel '}'
 
@@ -451,6 +464,12 @@ string_composite :: string string_composite?
  | bareword string_composite?
  | glob string_composite?
  | brace_expansion string_composite?
+ | heredoc_initiator string_composite? {append to heredoc_entries}
+
+heredoc_initiator :: '<' '<' '-' bareword {*bareword, interpolate, no deindent}
+ | '<' '<' '-' "'" [^']* "'" {*string, no interpolate, no deindent}
+ | '<' '<' '~' bareword {*bareword, interpolate, deindent}
+ | '<' '<' '~' "'" [^']* "'" {*bareword, no interpolate, deindent}
 
 string :: '"' dquoted_string_inner '"'
  | "'" [^']* "'"
@@ -476,7 +495,7 @@ comment :: '#' [^\n]*
 
 immediate_expression :: '$' '{' immediate_function expression* '}'
 
-immediate_function :: identifier { predetermined list of names, see Shell.h:ENUMERATE_SHELL_IMMEDIATE_FUNCTIONS }
+immediate_function :: identifier  { predetermined list of names, see Shell.h:ENUMERATE_SHELL_IMMEDIATE_FUNCTIONS }
 
 history_designator :: '!' event_selector (':' word_selector_composite)?