Merge branch 'ab/gc-docs'
Update docs around "gc".
* ab/gc-docs:
gc docs: remove incorrect reference to gc.auto=0
gc docs: clarify that "gc" doesn't throw away referenced objects
gc docs: note "gc --aggressive" in "fast-import"
gc docs: downplay the usefulness of --aggressive
gc docs: note how --aggressive impacts --window & --depth
gc docs: fix formatting for "gc.writeCommitGraph"
gc docs: re-flow the "gc.*" section in "config"
gc docs: include the "gc.*" section from "config" in "gc"
gc docs: clean grammar for "gc.bigPackThreshold"
gc docs: stop noting "repack" flags
gc docs: modernize the advice for manually running "gc"