Clarify the left- and right-flanking definitions (#534)

* Clarify the left- and right-flanking definitions

A few more words make the definitions easier to understand, without changing the meaning. 

Links to the current definitions:
- [left-flanking delimiter run](https://github.github.com/gfm/#left-flanking-delimiter-run)
- [right-flanking delimiter run](https://github.github.com/gfm/#right-flanking-delimiter-run)

* Change numbering to 1, 2a and 2b

1, 2a and 2b is more conventional than a, b1 and b2, which I suggested earlier.

spec.txt

@@ -5983,15 +5983,17 @@ of one or more `_` characters that is not preceded or followed by
 a non-backslash-escaped `_` character.
 
 A [left-flanking delimiter run](@) is
-a [delimiter run] that is (a) not followed by [Unicode whitespace],
-and (b) not followed by a [punctuation character], or
+a [delimiter run] that is (1) not followed by [Unicode whitespace],
+and either (2a) not followed by a [punctuation character], or
+(2b) followed by a [punctuation character] and
 preceded by [Unicode whitespace] or a [punctuation character].
 For purposes of this definition, the beginning and the end of
 the line count as Unicode whitespace.
 
 A [right-flanking delimiter run](@) is
-a [delimiter run] that is (a) not preceded by [Unicode whitespace],
-and (b) not preceded by a [punctuation character], or
+a [delimiter run] that is (1) not preceded by [Unicode whitespace],
+and either (2a) not preceded by a [punctuation character], or
+(2b) preceded by a [punctuation character] and
 followed by [Unicode whitespace] or a [punctuation character].
 For purposes of this definition, the beginning and the end of
 the line count as Unicode whitespace.