mk: Fix the regexp of SHOW_DOCS

The tag marks were missing, and `make tips` didn't work.
mk: Talk about `make clean` in the build help
auto merge of #12364 : Armavica/rust/mk-tips, r=alexcrichton

The command `make tips` did not work properly because of a flaw in the regexp parsing Makefile.in for documentation (`SHOW_DOCS`). I fixed it and also added a note about `make clean` in the build documentation (`make help`).