libsyntax: don't parse ////, /***/ as doc comments
auto merge of #4911 : lifthrasiir/rust/comment-exemptions, r=pcwalton

This is a natural extension of #4887, and handles the following three cases:

~~~~
a line with only /s
////////////////////////////////////////////

a line with only /s followed by whitespace
////////////////////////////////////////////         

a block comment with only *s between two /s
/********************************/
~~~~