Rollup merge of rust-lang#63373 - RalfJung:gitignore, r=alexcrichton

gitignore: add comment explaining policy

Based on rust-lang#63307 (comment), I added a comment what I think should be gitignored and what not. This is just a proposal, obviously.  Also see rust-lang#53768 for some more discussion.

The summary is that if there are junk files that you create locally and are fine leaving around (such as `mir_dump`), git has the option for you to add them to `.git/info/exclude`. Others might prefer to keep their working dir clean of those same junk files, so we shouldn't just ignore them for everyone.

I then also cleaned up a few more things, but there were many things that I had no idea where they came from so I didn't touch them.

.gitignore

@@ -1,3 +1,7 @@
+# This file should only ignore things that are generated during a build,
+# generated by common IDEs, and optional files controlled by the user
+# that affect the build (such as config.toml).
+# FIXME: This needs cleanup.
 *~
 .#*
 .DS_Store
@@ -14,20 +18,16 @@ __pycache__/
 .valgrindrc
 .vscode
 .favorites.json
-/*-*-*-*/
-/*-*-*/
 /Makefile
-/build
+/build/
 /config.toml
 /dist/
 /dl/
-/doc
+/doc/
 /inst/
 /llvm/
 /mingw-build/
-/nd/
 /obj/
-/rt/
 /rustllvm/
 /src/libcore/unicode/DerivedCoreProperties.txt
 /src/libcore/unicode/DerivedNormalizationProps.txt
@@ -37,11 +37,7 @@ __pycache__/
 /src/libcore/unicode/SpecialCasing.txt
 /src/libcore/unicode/UnicodeData.txt
 /src/libcore/unicode/downloaded
-/stage[0-9]+/
-/target
-target/
-/test/
-/tmp/
+/target/
 tags
 tags.*
 TAGS
@@ -50,17 +46,6 @@ TAGS.*
 \#*\#
 config.mk
 config.stamp
-keywords.md
-lexer.ml
 Session.vim
-src/etc/dl
-tmp.*.rs
-version.md
-version.ml
-version.texi
 .cargo
-!src/vendor/**
-/src/target/
-
 no_llvm_build
-