docs: describe ui.color consistently with --color stable

docs: describe ui.color consistently with --color

The --color option is described to accept "boolean, always, auto,
never, or debug". Let's use a similar description for ui.color. Also
fix the formatting to use double quotes as we seem to use for about
half the values in config.txt (the other half uses double
backticks). Also use uppercase Boolean for consistency within the
file.

--- a/mercurial/help/config.txt	Mon May 01 16:09:35 2017 +0200
+++ b/mercurial/help/config.txt	Mon May 01 11:04:10 2017 -0700
@@ -1860,11 +1860,9 @@
     By default, the first bundle advertised by the server is used.
 
 ``color``
-    String: when to use to colorize output. possible value are auto, always,
-    never, or debug (default: auto). 'auto' will use color whenever it seems
-    possible. See :hg:`help color` for details.
-
-    (in addition a boolean can be used in place always/never)
+    When to colorize output. Possible value are Boolean, "always", "auto",
+    "never", or "debug". (default: "auto"). "auto" will use color
+    whenever it seems possible. See :hg:`help color` for details.
 
 ``commitsubrepos``
     Whether to commit modified subrepositories when committing the