Document drracket:comment-delimiters

The documentation portion of #634.

Note that this commit does /not/ add to the documented list of keys
read by Dr Racket, because this commit does not add the behavior of Dr
Dracket doing that and using the comment styles to drive its
(un)comment commands.

drracket/info.rkt

@@ -67,7 +67,7 @@
 
 (define pkg-authors '(robby))
 
-(define version "1.14")
+(define version "1.15")
 
 (define license
   '(Apache-2.0 OR MIT))

drracket/scribblings/tools/lang-tools.scrbl

@@ -161,6 +161,75 @@ The default value is @racket[(list #\" #\|)].
  @history[#:added "1.10"]
 }
 
+@section{Comments}
+
+@language-info-def[drracket:comment-delimiters]{
+
+ When a language's @racket[_get-info] procedure responds to
+@racket['drracket:comment-delimiters], it is expected to return a
+value with this contract:
+
+ @racketblock[(listof
+               (or/c (list/c 'line string? string?)
+                     (list/c 'region string? string? string? string?)))]
+
+ The value is a list of comment styles. Each comment style is
+expressed as one of:
+
+ @itemlist[
+
+  @item{@racket[(list 'line start padding)], where @racket[_start]
+plus @racket[_padding] starts a comment that is teriminated by the end
+of a line.
+
+   Lisp example: @racket['(line ";;" " ")].
+
+   C++ example: @racket['(line "//" " ")].}
+
+ @item{@racket[(list 'region start continue end padding)], where:
+
+   @itemlist[
+
+    @item{@racket[_start] then @racket[_padding] opens a comment}
+
+    @item{@racket[_continue] then @racket[padding] is added to the
+beginning of each line except the first one when a comment spans
+multiple lines}
+
+    @item{@racket[_padding] then @racket[_end] closes a comment}]
+
+   Racket example: @racket['(region "#|" "|#" "  " " ")].
+
+   C++ example: @racket['(region "/*" " *" "*/" " ")].}
+
+ ]
+
+ When not specified by a lang, the default value is suitable for
+Racket s-expression langs:
+
+ @racketblock['((line ";;" " ")
+                (region "#|" "|#" "  " " "))]
+
+ An intended use for these values is by (un)comment commands, which
+vary among tools. Some tools (un)comment entire lines, whereas others
+may handle portions of a line. Generally this is orthogonal to using a
+lang's line vs. region style: A tool can wrap entire lines using
+region comments. A tool can insert line breaks to make it possible to
+use line comments on a portion of a line. The point of
+@racket['drracket:comment-delimiters] is to enable a lang to tell a
+tool about its comment delimiters --- not to say exactly how
+the (un)comment commands could or should work, exactly.
+
+ When the list has multiple styles: Some tools may present the styles
+for the user to pick one. Other tools may default to using the first
+style in the list (allowing the user to configure another preference
+by other means). Therefore when a language supports multiple comment
+styles, it should @emph{list the most popular or preferred style
+first}.
+
+ @history[#:added "1.15"]
+}
+
 @section{Keystrokes}
 
 @language-info-def[drracket:keystrokes]{