Document theme attribute in diff preferences info entity

Ic97c237d7 added support for CodeMirror themes in SBS2, but missed to
document theme attribute in diff preferences info entity.

Change-Id: I7cd5ea519b8b8491138192053bfe7be09ba741d2
diff --git a/Documentation/rest-api-accounts.txt b/Documentation/rest-api-accounts.txt
index 24da5e4..1fe09e7 100644
--- a/Documentation/rest-api-accounts.txt
+++ b/Documentation/rest-api-accounts.txt
@@ -1024,6 +1024,7 @@
   )]}'
   {
     "context": 10,
+    "theme": "DEFAULT",
     "ignore_whitespace": "IGNORE_ALL_SPACE",
     "intraline_difference": true,
     "line_length": 100,
@@ -1052,6 +1053,7 @@
 
   {
     "context": 10,
+    "theme": "ECLIPSE",
     "ignore_whitespace": "IGNORE_ALL_SPACE",
     "intraline_difference": true,
     "line_length": 100,
@@ -1075,6 +1077,7 @@
   )]}'
   {
     "context": 10,
+    "theme": "ECLIPSE",
     "ignore_whitespace": "IGNORE_ALL_SPACE",
     "intraline_difference": true,
     "line_length": 100,
@@ -1421,6 +1424,9 @@
 |Field Name                    ||Description
 |`context`                     ||
 The number of lines of context when viewing a patch.
+|`theme`                       ||
+The CodeMirror theme. Currently only a subset of light and dark
+CodeMirror themes are supported.
 |`expand_all_comments`         |not set if `false`|
 Whether all inline comments should be automatically expanded.
 |`ignore_whitespace`           ||