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 <ray@ameretat.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-apply.txt

@@ -61,18 +61,18 @@ OPTIONS
 	file and detects errors.  Turns off "apply".
 
 --index::
-	When `--check` is in effect, or when applying the patch
+	Apply the patch to both the index and the working tree (or
-	(which is the default when none of the options that
+	merely check that it would apply cleanly to both if `--check` is
-	disables it is in effect), make sure the patch is
+	in effect). Note that `--index` expects index entries and
-	applicable to what the current index file records.  If
+	working tree copies for relevant paths to be identical (their
-	the file to be patched in the working tree is not
+	contents and metadata such as file mode must match), and will
-	up to date, it is flagged as an error.  This flag also
+	raise an error if they are not, even if the patch would apply
-	causes the index file to be updated.
+	cleanly to both the index and the working tree in isolation.
 
 --cached::
-	Apply a patch without touching the working tree. Instead take the
+	Apply the patch to just the index, without touching the working
-	cached data, apply the patch, and store the result in the index
+	tree. If `--check` is in effect, merely check that it would
-	without using the working tree. This implies `--index`.
+	apply cleanly to the index entry.
 
 --intent-to-add::
 	When applying the patch only to the working tree, mark new