Update documentation for definition_list extension.

In 2015, we relaxed indentation requirements for the first
line of a definition (see commit d3544dc and issue #2087), but
the documnentation wasn't updated to reflect the change.

Closes #7594.

MANUAL.txt

@@ -3749,8 +3749,8 @@ or two spaces.
 A term may have multiple definitions, and each definition may
 consist of one or more block elements (paragraph, code block,
 list, etc.), each indented four spaces or one tab stop.  The
-body of the definition (including the first line, aside from the
-colon or tilde) should be indented four spaces. However, as with
+body of the definition (not including the first line)
+should be indented four spaces. However, as with
 other Markdown lists, you can "lazily" omit indentation except
 at the beginning of a paragraph or other block element: