sparserevlog: document the config option stable

sparserevlog: document the config option

This was overlooked when this graduated from experimental.

--- a/mercurial/help/config.txt	Wed Jan 23 07:49:36 2019 -0500
+++ b/mercurial/help/config.txt	Wed Jan 30 18:15:38 2019 +0100
@@ -857,6 +857,16 @@
 
     Enabled by default.
 
+``sparse-revlog``
+    Enable or disable the ``sparse-revlog`` delta strategy. This format improves
+    delta re-use inside revlog. For very branchy repositories, it results in a
+    smaller store. For repositories with many revisions, it also helps
+    performance (by using shortened delta chains.)
+
+    Repositories with this on-disk format require Mercurial version 4.7
+
+    Enabled by default.
+
 ``graph``
 ---------
 

--- a/mercurial/localrepo.py	Wed Jan 23 07:49:36 2019 -0500
+++ b/mercurial/localrepo.py	Wed Jan 30 18:15:38 2019 +0100
@@ -2920,7 +2920,6 @@
 
     if scmutil.gdinitconfig(ui):
         requirements.add('generaldelta')
-        # experimental config: format.sparse-revlog
         if ui.configbool('format', 'sparse-revlog'):
             requirements.add(SPARSEREVLOG_REQUIREMENT)
     if ui.configbool('experimental', 'treemanifest'):

--- a/tests/test-help.t	Wed Jan 23 07:49:36 2019 -0500
+++ b/tests/test-help.t	Wed Jan 30 18:15:38 2019 +0100
@@ -1502,6 +1502,8 @@
   
       "usestore"
   
+      "sparse-revlog"
+  
       "profiling"
       -----------