help: add information about recovery from corruption to help of "verify"

Before this patch, there is no information about what users should (or
can) do for recovery from corruption of repositories.

This patch adds URL of the Mercurial Wiki page explaining about
recovery from corruption.

mercurial/commands.py

@@ -5879,6 +5879,10 b' def verify(ui, repo):'
     the changelog, manifest, and tracked files, as well as the
     integrity of their crosslinks and indices.
 
+    Please see http://mercurial.selenic.com/wiki/RepositoryCorruption
+    for more information about recovery from corruption of the
+    repository.
+
     Returns 0 on success, 1 if errors are encountered.
     """
     return hg.verify(repo)

tests/test-help.t

@@ -392,6 +392,9 b' Test command without options'
       manifest, and tracked files, as well as the integrity of their crosslinks
       and indices.
   
+      Please see http://mercurial.selenic.com/wiki/RepositoryCorruption for more
+      information about recovery from corruption of the repository.
+  
       Returns 0 on success, 1 if errors are encountered.
   
   use "hg -v help verify" to show more info