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"