help: describe about choice of :prompt as a fallback merge tool explicitly

"merge-tools" help topic has described that the merge of the file
fails if no tool is found to merge binary or symlink, since
c77f6276c9e7 (or Mercurial 1.7), which based on (already removed)
MergeProgram wiki page.

But even at that revision, and of course now, merge of the file
doesn't fail automatically for binary/symlink. ":prompt" (or
equivalent logic) is used, if there is no appropriate tool
configuration for binary/symlink.

mercurial/help/merge-tools.txt

@@ -70,7 +70,7 b' 6. If a program named ``hgmerge`` can be'
 7. If the file to be merged is not binary and is not a symlink, then
    internal ``:merge`` is used.
 
-8. The merge of the file fails and must be resolved before commit.
+8. Otherwise, ``:prompt`` is used.
 
 .. note::
 

tests/test-help.t

@@ -1831,7 +1831,7 b' Test dynamic list of merge tools only sh'
          but it will by default not be used for symlinks and binary files.
       7. If the file to be merged is not binary and is not a symlink, then
          internal ":merge" is used.
-      8. The merge of the file fails and must be resolved before commit.
+      8. Otherwise, ":prompt" is used.
   
       Note:
          After selecting a merge program, Mercurial will by default attempt to