convert: more clear documentation of the 'include' default of a 'include .'

At first glance it can be confusing that adding a superfluous include directive
will exclude more files.

hgext/convert/__init__.py

@@ -101,9 +101,10 b' def convert(ui, src, dest=None, revmapfi'
     longest matching path applies, so line order does not matter.
 
     The ``include`` directive causes a file, or all files under a
-    directory, to be included in the destination repository, and the
-    exclusion of all other files and directories not explicitly
-    included. The ``exclude`` directive causes files or directories to
+    directory, to be included in the destination repository. The default
+    if there are no ``include`` statements is to include everything.
+    If there are any ``include`` statements, nothing else is included.
+    The ``exclude`` directive causes files or directories to
     be omitted. The ``rename`` directive renames a file or directory if
     it is converted. To rename from a subdirectory into the root of
     the repository, use ``.`` as the path to rename to.

tests/test-convert.t

@@ -84,11 +84,12 b''
       so line order does not matter.
   
       The "include" directive causes a file, or all files under a directory, to
-      be included in the destination repository, and the exclusion of all other
-      files and directories not explicitly included. The "exclude" directive
-      causes files or directories to be omitted. The "rename" directive renames
-      a file or directory if it is converted. To rename from a subdirectory into
-      the root of the repository, use "." as the path to rename to.
+      be included in the destination repository. The default if there are no
+      "include" statements is to include everything. If there are any "include"
+      statements, nothing else is included. The "exclude" directive causes files
+      or directories to be omitted. The "rename" directive renames a file or
+      directory if it is converted. To rename from a subdirectory into the root
+      of the repository, use "." as the path to rename to.
   
       The splicemap is a file that allows insertion of synthetic history,
       letting you specify the parents of a revision. This is useful if you want