Fix $anchor plain name format to match XML's NCName production

[ssilverman]

This uses the US-ASCII part of the production.
See: https://www.w3.org/TR/2006/REC-xml-names11-20060816/#NT-NCName

[handrews]

@awwright I think you're the only person who would remember why the existing rules were done the way they were. If you are OK with this, I'm OK with this.

In section 5: Fragments, we reference the W3C best practices document, which at one point talks about the NCName production in the context of plain name fragments.

Whatever we do, section 5 and the section for $anchor need to agree.

[awwright]

Compatibility with XML and ASCII (because of the 7-bit nature of URIs, at least until we decide to commit to IRIs) were the only considerations, iirc.

For reference:

Name ::= NameStartChar (NameChar)*
NameStartChar ::= ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
NameChar ::= NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]

[ssilverman]

Just to be certain, the formatting and use of eref is okay? I'm thinking of instead wrapping that around some text. Do you guys have a style preference?

Update: On second thought, I like it how it is. I just force pushed, but I only added a period to end the sentence.

[handrews]

@ssilverman I'd make an <xref...>NCName production</xref> for better readability. I can't remember if that's already declared as an external document at the top or and bottom, but if not there are other W3C specs referenced- you'll figure it out :-)

You can't include an anchor in that reference, but "NCName production" will be enough for anyone to locate it within the spec.

[ssilverman]

Updated to xref.

[handrews]

@ssilverman you're getting a build error through Travis. I think you need to add a corresponding <!ENTITY declaration at the top. xrefs to external specifications require both that and an entry under references.

[ssilverman]

I added a missing "date" element; that seems to be what the DTD wanted. I also updated the target link because if the date is needed, then we need a link having a date that matches, not future revisions.