Auto merge of #28174 - steveklabnik:gh14705, r=alexcricton

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

mk/docs.mk

@@ -191,6 +191,7 @@ ifdef CFG_DISABLE_DOCS
 endif
 
 docs: $(DOC_TARGETS)
+doc: docs
 compiler-docs: $(COMPILER_DOC_TARGETS)
 
 trpl: doc/book/index.html