man: Document DPKG_GENSYMBOLS_CHECK_LEVEL in dpkg-gensymbols(1)

debian/changelog

@@ -4,6 +4,8 @@ dpkg (1.18.16) UNRELEASED; urgency=medium
   * Documentation:
     - State the current working directory used when running dpkg-buildpackage
       hooks.
+    - Add a new ENVIRONMENT section to dpkg-gensymbols(1) and document
+      DPKG_GENSYMBOLS_CHECK_LEVEL there.
 
   [ Updated scripts translations ]
   * German (Helge Kreutzmann).

man/dpkg-gensymbols.man

@@ -501,6 +501,12 @@ Show the usage message and exit.
 .BR \-\-version
 Show the version and exit.
 .
+.SH ENVIRONMENT
+.TP
+.B DPKG_GENSYMBOLS_CHECK_LEVEL
+Overrides the command check level, even if the \fB\-c\fP command-line
+argument was given (note that this goes against the common convention
+of command-line arguments having precedence over environment variables).
 .SH SEE ALSO
 .BR https://people.redhat.com/drepper/symbol\-versioning
 .br