man: Document that dpkg performs sanity checks on PATH

debian/changelog

@@ -117,6 +117,7 @@ dpkg (1.18.0) UNRELEASED; urgency=low
       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.
+    - Document that dpkg performs sanity checks on PATH.
   * 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.1

@@ -748,6 +748,11 @@ Cancels a previous \fB\-\-no\-triggers\fP (since dpkg 1.14.17).
 .
 .SH ENVIRONMENT
 .TP
+.B PATH
+This variable is expected to be defined in the environment and point to
+the system paths where several required programs are to be found. If it's
+not set or the programs are not found, \fBdpkg\fP will abort.
+.TP
 .B HOME
 If set, \fBdpkg\fP will use it as the directory from which to read the user
 specific configuration file.