[lex.header] Modernize text around header names

[AlisdairM]

The footnote better belongs in the main text as a regular note.
To make the notes flow consistently, switch the order of the
note and normative text in the first paragraph to lead with the
normative text.

[AlisdairM]

Flagging that I believe I have addressed the changes requested by Jens.

[tkoeppe]

Editorial meeting discussion in Wroclaw 2024:

There does not seem to be a simple, context-free rule for how to spell characters (such as "always use the unicode named reference"). There cases where we definitely want the named reference, such as in the lexer grammar. There are also cases where the literal character is most appropriate, such as in "the unary & operator". In the present proposed change, having / adjacent to the two-character /*, which we'd never consider spelling as a sequence of named references, seems to be entirely clear.

Therefore, we don't want any kind of context-free blanket replacement of characters with named unicode references, or vice versa. It is possible that we can identify a more nuanced rule when to apply.

[AlisdairM]

Removed the last part of the Unicode markup changes from the original commit message:
" Finally, take this opportunity to clarify the specific characters called out using the Unicode nomenclature used throughout this clause."