Document multi-mode indentation facilities

* doc/lispref/text.texi (Mode-Specific Indent): Document
'prog-indentation-context', 'prog-first-column', and 'prog-widen'.

* lisp/progmodes/prog-mode.el (prog-indentation-context)
(prog-widen): Doc fixes.

1 files changed, 10 insertions, 1 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 01447cde163..246ee37a848 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -321,8 +321,17 @@ the ordering of object keys by default.
 `json-pretty-print-buffer-ordered' pretty prints JSON objects with
 object keys sorted alphabetically.
 
++++
 ** Prog mode has some support for multi-mode indentation.
-See `prog-indentation-context' and `prog-widen'.
+This allows better indentation support in modes that support multiple
+programming languages in the same buffer, like literate programming
+environments or ANTLR programs with embedded Python code.
+
+A major mode can provide indentation context for a sub-mode through
+the `prog-indentation-context' variable.  To support this, modes that
+provide indentation should use `prog-widen' instead of `widen' and
+`prog-first-column' instead of a literal zero.  See the node
+"Mode-Specific Indent" in the ELisp manual for more details.
 
 ** Prettify Symbols mode