man: Document in dpkg-buildflags(1) that DEB_VENDOR influences the execution

debian/changelog

@@ -116,6 +116,7 @@ dpkg (1.18.0) UNRELEASED; urgency=low
     - Document in dpkg-buildpackage(1) that using dpkg-source is sometimes
       better than -S. Suggested by Johannes Schauer <j.schauer@email.de>.
     - Document dselect -? option.
+    - Document in dpkg-buildflags(1) that DEB_VENDOR influences the execution.
   * Output message fixes and improvements:
     - Remove trailing newline from string literal in warning calls.
     - Say “execute” instead of “exec” in Dpkg::Changelog::Parse error message.

man/dpkg-buildflags.1

@@ -431,6 +431,12 @@ User configuration file.
 Makefile snippet that will load (and optionally export) all flags
 supported by \fBdpkg-buildflags\fP into variables (since dpkg 1.16.1).
 .
+.SH ENVIRONMENT
+.TP
+.B DEB_VENDOR
+This setting defines the current vendor. If not set, it will discover the
+current vendor by reading \fB/etc/dpkg/origins/default\fP.
+.
 .SH EXAMPLES
 To pass build flags to a build command in a makefile:
 .PP