Fix some typos, misformattings and small mistakes in the lexical structure reference.

[steffahn]

Multiple unrelated things in one commit since they’re each very small corrections.
Next to typos and bad formatting, this PR changes a word (“equivalent” ⟶ “equal”) and adds (more precisely, disallows) “\n” in a few obvious places in comments’ lexical structure.

[ehuss]

Thanks!

[steffahn]

Just noticing, perhaps the IsolatedCR should be disallowed in the front part of OUTER_LINE_DOC, too...
Furthermore – but on this part I’m not sure about what exactly \n and IsolatedCR mean here – should IsolatedCR also be disallowed in LINE_COMMENT?

[steffahn]

Hm.. looking at it further, so far IsolatedCR is only forbidden in anything “DOC” comment, so perhaps it is allowed within a LINE_COMMENT. [Edit: tested it, it is allowed, also the paragraph below mentions CRs in doc comments, I just noticed.] But I found another mistake in the beginning of OUTER_BLOCK_DOC where the ~* most likely needs an IsolatedCR excluded, too. Will add this to the commit aswell.

[ehuss]

Yea, that seems correct.

Also, just FYI, there are a number of problems with the lexer chapter. CRLF handling was recently rewritten, which hasn't been updated in the reference. See #626. Also, I've been meaning to rewrite the lexer chapter, since it is a bit disorganized right now (#567 mentions some of the problems). If you're interested in working on those (or any issues), that would be great!

(I'll leave this PR open for a bit to see if @Centril wants to look.)

[steffahn]

Another thing I noticed: The whole thing with BlockCommentOrDoc is totally wrong regarding IsolatedCR s. As far as a few quick tests are telling me, what actually matters is if the whole comment is a block comment or not, not if nested comments happen to be syntactically like a doc comment or a normal comment.
Finally, and this may just be me not quite getting how these grammars are supposed to be read regarding maximum munch etc... but I feel like I’m missing that /* be disallowed, too, inside the BLOCK_COMMENT rules where it currently says “~*/”.

[Centril]

(I'll leave this PR open for a bit to see if @Centril wants to look.)

cc @petrochenkov @matklad -- I'm not too familiar with the lexer myself.