git-apply.txt: update descriptions of --cached, --index

git-apply.txt: update descriptions of --cached, --index

The blurb for "--cached" says it implies "--index", but in reality
"--cached" and "--index" are distinct modes with different behavior.

Additionally, the descriptions of "--index" and "--cached" are somewhat
unclear about what might be modified, and what "--index" looks for to
determine that the index and working copy "match".

Rewrite the blurbs for both options for clarity and accuracy.

Signed-off-by: Raymond E. Pasco <redacted>
Signed-off-by: Junio C Hamano <redacted>