INSTALL: Add MANPATH instructions

INSTALL.in

@@ -20,5 +20,17 @@ set the environment variable GIT_EXEC_PATH accordingly:
   $ export "GIT_EXEC_PATH=@pkglibexecdir@:$(git --exec-path)"
   $ git --exec-path
   @pkglibexecdir@:/usr/libexec/git-core
-  $ git amb --help # should come up with the git-amb(1) man page
+  $ git amb --version
+  git-amb (ndims git utilities) 1.22.1-64ea8c7
+
+A similar operation is required to set up the man pages so that both
+"man git-amb" and "git amb --help" can display the git-amb(1) man
+page:
 
+  $ manpath
+  /usr/local/share/man:/usr/share/man
+  $ export MANPATH="@mandir@:$(manpath)"
+  $ manpath
+  manpath: warning: $MANPATH set, ignoring /etc/man_db.conf
+  @mandir@:/usr/local/share/man:/usr/share/man
+  $ git amb --help # should come up with the git-amb(1) man page

Makefile.am

@@ -47,8 +47,9 @@ CLEANFILES += INSTALL
 EXTRA_DIST += INSTALL.in
 INSTALL: $(top_srcdir)/INSTALL.in Makefile
 	$(SED) \
-		-e 's,[@]prefix@,$(prefix),g' \
+		-e 's,[@]mandir@,$(mandir),g' \
 		-e 's,[@]pkglibexecdir@,$(pkglibexecdir),g' \
+		-e 's,[@]prefix@,$(prefix),g' \
 		$(top_srcdir)/INSTALL.in > INSTALL
 
 dist-hook: dist-hook-rm-INSTALL-file