config: fix commit-graph related config docs

The core.commitGraph config setting was accidentally removed from
the config documentation. In that same patch, the config setting
that writes a commit-graph during garbage collection was incorrectly
written to the doc as "gc.commitGraph" instead of "gc.writeCommitGraph".

Reported-by: Szeder Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -904,12 +904,10 @@ core.notesRef::
 This setting defaults to "refs/notes/commits", and it can be overridden by
 the `GIT_NOTES_REF` environment variable.  See linkgit:git-notes[1].
-gc.commitGraph::
-	If true, then gc will rewrite the commit-graph file when
-	linkgit:git-gc[1] is run. When using linkgit:git-gc[1]
-	'--auto' the commit-graph will be updated if housekeeping is
-	required. Default is false. See linkgit:git-commit-graph[1]
-	for details.
+core.commitGraph::
+	If true, then git will read the commit-graph file (if it exists)
+	to parse the graph structure of commits. Defaults to false. See
+	linkgit:git-commit-graph[1] for more information.
 core.sparseCheckout::
 	Enable "sparse checkout" feature. See section "Sparse checkout" in
@@ -1651,6 +1649,13 @@ this configuration variable is ignored, all packs except the base pack
 will be repacked. After this the number of packs should go below
 gc.autoPackLimit and gc.bigPackThreshold should be respected again.
+gc.writeCommitGraph::
+	If true, then gc will rewrite the commit-graph file when
+	linkgit:git-gc[1] is run. When using linkgit:git-gc[1]
+	'--auto' the commit-graph will be updated if housekeeping is
+	required. Default is false. See linkgit:git-commit-graph[1]
+	for details.
+
 gc.logExpiry::
 	If the file gc.log exists, then `git gc --auto` won't run
 	unless that file is more than 'gc.logExpiry' old.  Default is