doc: Add reference for ‘git-version’ and ‘hg-version’.

* doc/contributing.texi (Version Numbers): Document ‘git-version’ and
  ‘hg-version’.
* doc/guix.texi (package Reference): Reference ‘Version Numbers’ section for
  version naming guidelines.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

2 files changed, 22 insertions, 4 deletions

diff --git a/doc/contributing.texi b/doc/contributing.texi
index fa8c0cf3ba..d67e632520 100644
--- a/doc/contributing.texi
+++ b/doc/contributing.texi
@@ -532,9 +532,11 @@ It is a good idea to strip commit identifiers in the @code{version}
 field to, say, 7 digits.  It avoids an aesthetic annoyance (assuming
 aesthetics have a role to play here) as well as problems related to OS
 limits such as the maximum shebang length (127 bytes for the Linux
-kernel).  It is best to use the full commit identifiers in
-@code{origin}s, though, to avoid ambiguities.  A typical package
-definition may look like this:
+kernel).  There are helper functions for doing this for packages using
+@code{git-fetch} or @code{hg-fetch} (see below).  It is best to use the
+full commit identifiers in @code{origin}s, though, to avoid ambiguities.
+A typical package definition may look like this:
+
 
 @lisp
 (define my-package
@@ -553,6 +555,20 @@ definition may look like this:
       )))
 @end lisp
 
+@deffn {Scheme Procedure} git-version @var{VERSION} @var{REVISION} @var{COMMIT}
+Return the version string for packages using @code{git-fetch}.
+
+@lisp
+(git-version "0.2.3" "0" "93818c936ee7e2f1ba1b315578bde363a7d43d05")
+@result{} "0.2.3-0.93818c9"
+@end lisp
+@end deffn
+
+@deffn {Scheme Procedure} hg-version @var{VERSION} @var{REVISION} @var{CHANGESET}
+Return the version string for packages using @code{hg-fetch}.  It works
+in the same way as @code{git-version}.
+@end deffn
+
 @node Synopses and Descriptions
 @subsection Synopses and Descriptions
 
diff --git a/doc/guix.texi b/doc/guix.texi
index eb64518a95..e71ec70859 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -89,6 +89,7 @@ Copyright @copyright{} 2020 Jonathan Brielmaier@*
 Copyright @copyright{} 2020 Edgar Vincent@*
 Copyright @copyright{} 2021 Maxime Devos@*
 Copyright @copyright{} 2021 B. Wilson@*
+Copyright @copyright{} 2021 Xinglu Chen@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -6657,7 +6658,8 @@ This is the data type representing a package recipe.
 The name of the package, as a string.
 
 @item @code{version}
-The version of the package, as a string.
+The version of the package, as a string.  @xref{Version Numbers}, for
+guidelines.
 
 @item @code{source}
 An object telling how the source code for the package should be