document that multiple groupwanted are not combined

doc/forum/Multiple_group_groupwanted_settings_help/comment_1_e78d0aa6b23225c509b9e97549fe915b._comment

@@ -0,0 +1,34 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2018-06-12T16:17:30Z"
+ content="""
+groupwanted expressions are only used when there is only one that the
+repository could use. The repository can be in two groups and if only one
+of the groups has a groupwanted expression, it will be used, but if both
+do, neither expression will be used. 
+
+This limitation to exactly one expression also holds for standard
+preferred content expressions (which is documented).
+git-annex does not try to combine together two expressions with OR.
+Why not? Well, consider the standard preferred content expressions for the 
+client and transfer groups. The one for transfer looks at whether client
+repositories have the content:
+`not (inallgroup=client and copies=client:2) ...`
+
+If a repository is both a transfer repository and a client at the same
+time, that would make it want to have content as long as not all clients
+contain the content -- so once all other clients get the content, the
+repository would want to drop its copy. But then, it would see that
+not all client repositories now have the content (since it doesn't!) and
+want to get it. And potentially round and round.. 
+
+git-annex actually detects and prevents this kind of drop/get cycle
+(probably), but the right behavior of combining those two groupwanted
+expressions is at best undefined. So it's better to have the user write
+down an expression with what they really want to happen than try to OR
+expressions.
+
+I've updated the docs for groupwanted to note that it's only used when one 
+of a repos's groups has it.
+"""]]

doc/git-annex-groupwanted.mdwn

@@ -21,6 +21,10 @@ make repositories in the group want to contain 3 copies of every file:
 		git annex wanted $repo groupwanted
 	done
 
+Note that there must be exactly one groupwanted expression configured
+amoung all the groups that a repository is in; if there's more than one,
+none of them will be used.
+
 # SEE ALSO
 
 [[git-annex]](1)

doc/git-annex-preferred-content.mdwn

@@ -172,12 +172,13 @@ elsewhere to allow removing it).
 * `groupwanted`
 
   The "groupwanted" keyword can be used to refer to a preferred content
-  expression that is associated with a group. This is like the "standard"
-  keyword, but you can configure the preferred content expressions
-  using `git annex groupwanted`.
+  expression that is associated with a group, as long as there is exactly
+  one such expression amoung the groups a repository is in. This is like
+  the "standard" keyword, but you can configure the preferred content
+  expressions using `git annex groupwanted`.
 
-  Note that when writing a groupwanted preferred content expression,
-  you can use all of the keywords listed above, including "standard".
+  When writing a groupwanted preferred content expression,
+  you can use all the keywords documented here, including "standard".
   (But not "groupwanted".)
 
   For example, to make a variant of the standard client preferred content