help: describe more detail about capabilities while deciding merge tool

"hg help merge-tools" describes as below:

    (internal merge tools) will by default not handle symlinks or
    binary files.

But in some cases, Mercurial assumes that internal merge tools have
one or both of these capabilities.

"hg help merge-tools" also describes as below, for matching patterns in
merge-patterns configuration section. But this is not sufficient.

    Here, binary capabilities of the merge tool are not considered.

This patch describes more detail about capabilities while deciding
merge tool.

mercurial/help/merge-tools.txt

@@ -54,8 +54,7 b' 2. If the ``HGMERGE`` environment variab'
 
 3. If the filename of the file to be merged matches any of the patterns in the
    merge-patterns configuration section, the first usable merge tool
-   corresponding to a matching pattern is used. Here, binary capabilities of the
-   merge tool are not considered.
+   corresponding to a matching pattern is used.
 
 4. If ui.merge is set it will be considered next. If the value is not the name
    of a configured tool, the specified value is used and must be executable by
@@ -72,6 +71,19 b' 7. If the file to be merged is not binar'
 
 8. Otherwise, ``:prompt`` is used.
 
+For historical reason, Mercurial assumes capabilities of internal
+merge tools as below while examining rules above, regardless of actual
+capabilities of them.
+
+==== =============== ====== =======
+step specified via   binary symlink
+==== =============== ====== =======
+1.   --tool          o      o
+2.   HGMERGE         o      o
+3.   merge-patterns  o      x
+4.   ui.merge        x      x
+==== =============== ====== =======
+
 .. note::
 
    After selecting a merge program, Mercurial will by default attempt

tests/test-help.t

@@ -1892,8 +1892,7 b' Test dynamic list of merge tools only sh'
          must be executable by the shell.
       3. If the filename of the file to be merged matches any of the patterns in
          the merge-patterns configuration section, the first usable merge tool
-         corresponding to a matching pattern is used. Here, binary capabilities
-         of the merge tool are not considered.
+         corresponding to a matching pattern is used.
       4. If ui.merge is set it will be considered next. If the value is not the
          name of a configured tool, the specified value is used and must be
          executable by the shell. Otherwise the named tool is used if it is
@@ -1906,6 +1905,17 b' Test dynamic list of merge tools only sh'
          internal ":merge" is used.
       8. Otherwise, ":prompt" is used.
   
+      For historical reason, Mercurial assumes capabilities of internal merge
+      tools as below while examining rules above, regardless of actual
+      capabilities of them.
+  
+      step specified via  binary symlink
+      ----------------------------------
+      1.   --tool         o      o
+      2.   HGMERGE        o      o
+      3.   merge-patterns o      x
+      4.   ui.merge       x      x
+  
       Note:
          After selecting a merge program, Mercurial will by default attempt to
          merge the files using a simple merge algorithm first. Only if it