doc lint: make "lint-docs" non-.PHONY

Speed up the "lint-docs" target by making it non-.PHONY. Similar to my c234e8a0ecf (Makefile: make the "sparse" target non-.PHONY, 2021-09-23). We'll now create empty files corresponding to a dependency graph for each of these lint scripts. This speeds things up a bit[1], and makes the output correspond to any in-tree changes we have: $ touch git-add.txt; make lint-docs; make lint-docs GEN cmd-list.made GEN doc.dep LINT GITLINK git-add.txt LINT MAN END git-add.txt LINT MAN SEC git-add.txt make: Nothing to be done for 'lint-docs'. As with the "sparse" target changes this has a hard dependency on the use of ".DELETE_ON_ERROR" in the Makefile, added here in db10fc6c09f (doc: simplify Makefile using .DELETE_ON_ERROR, 2021-05-21). This method also depends on the output for us emitting any errors on STDERR (fixed in a preceding commit), as well us these scripts exiting with non-zero on any errors (which they were already doing). 1. $ git show HEAD~:Documentation/Makefile >Makefile.old $ hyperfine --warmup 2 -L f ",.old" 'make -j1 -f Makefile{f} lint-docs' Benchmark #1: make -j1 -f Makefile lint-docs Time (mean ± σ): 60.8 ms ± 1.4 ms [User: 58.7 ms, System: 2.5 ms] Range (min … max): 58.9 ms … 64.0 ms 48 runs Benchmark #2: make -j1 -f Makefile.old lint-docs Time (mean ± σ): 84.0 ms ± 1.5 ms [User: 78.6 ms, System: 5.7 ms] Range (min … max): 81.8 ms … 87.8 ms 35 runs Summary 'make -j1 -f Makefile lint-docs' ran 1.38 ± 0.04 times faster than 'make -j1 -f Makefile.old lint-docs' Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>