Introduce 'make doc' -> 'make docs'

Because 'doc' is a directory, when running `make doc`, you'll see
this:

    make: Nothing to be done for `doc'.

By adding a target for `doc` to build `docs`, both work.

Fixes #14705

Makefile.in

@@ -267,3 +267,5 @@ ifneq ($(strip $(findstring TAGS.emacs,$(MAKECMDGOALS)) \
   CFG_INFO := $(info cfg: including ctags rules)
   include $(CFG_SRC_DIR)mk/ctags.mk
 endif
+
+doc: docs