docs: rephrase and clarify the git status --short format

The table describing the porcelain format in git-status(1) is helpful,
but it's not completely clear what the three sections mean, even to
some contributors.  As a result, users are unable to find how to detect
common cases like merge conflicts programmatically.

Let's improve this situation by rephrasing to be more explicit about
what each of the sections in the table means, to tell users in plain
language which cases are occurring, and to describe what "unmerged"
means.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

Documentation/git-status.txt

@@ -184,11 +184,26 @@ characters, that field will be quoted in the manner of a C string
 literal: surrounded by ASCII double quote (34) characters, and with
 interior special characters backslash-escaped.
 
-For paths with merge conflicts, `X` and `Y` show the modification
-states of each side of the merge. For paths that do not have merge
-conflicts, `X` shows the status of the index, and `Y` shows the status
-of the work tree.  For untracked paths, `XY` are `??`.  Other status
-codes can be interpreted as follows:
+There are three different types of states that are shown using this format, and
+each one uses the `XY` syntax differently:
+
+* When a merge is occurring and the merge was successful, or outside of a merge
+	situation, `X` shows the status of the index and `Y` shows the status of the
+	working tree.
+* When a merge conflict has occurred and has not yet been resolved, `X` and `Y`
+	show the state introduced by each head of the merge, relative to the common
+	ancestor. These paths are said to be _unmerged_.
+* When a path is untracked, `X` and `Y` are always the same, since they are
+	unknown to the index. `??` is used for untracked paths. Ignored files are
+	not listed unless `--ignored` is used; if it is, ignored files are indicated
+	by `!!`.
+
+Note that the term _merge_ here also includes rebases using the default
+`--merge` strategy, cherry-picks, and anything else using the merge machinery.
+
+In the following table, these three classes are shown in separate sections, and
+these characters are used for `X` and `Y` fields for the first two sections that
+show tracked paths:
 
 * ' ' = unmodified
 * 'M' = modified
@@ -198,9 +213,6 @@ codes can be interpreted as follows:
 * 'C' = copied
 * 'U' = updated but unmerged
 
-Ignored files are not listed, unless `--ignored` option is in effect,
-in which case `XY` are `!!`.
-
 ....
 X          Y     Meaning
 -------------------------------------------------