dpkg-gensymbols(1): document syntax of comments

Reported-by: Sven Joachim <svenjoac@gmx.de>

debian/changelog

@@ -44,6 +44,7 @@ dpkg (1.16.1) UNRELEASED; urgency=low
     in dpkg-source(1). Closes: #631435
   * Add some supplementary checks to ensure debian/control has the required
     fields. Closes: #631439
+  * dpkg-gensymbols(1): document syntax of comments. Closes: #630996
 
   [ Guillem Jover ]
   * Install deb-src-control(5) man pages in dpkg-dev. Closes: #620520

man/dpkg-gensymbols.1

@@ -56,6 +56,11 @@ with "~".
 Before applying any patch to the symbols file, the maintainer should
 double-check that it's sane. Public symbols are not supposed to disappear,
 so the patch should ideally only add new lines.
+.P
+Note that you can put comments in symbols files: any line with '#' as the
+first character is a comment except if it starts with '#include' (see
+section \fBUsing includes\fP). Lines starting with '#MISSING:' are special
+comments documenting symbols that have disappeared.
 .SS Using #PACKAGE# substitution
 .P
 In some rare cases, the name of the library varies between architectures.