header: typo fix in a comment

Reviewed-By: Fedor Indutny <fedor@indutny.com>
doc: add very basic docs for `http_parser_execute`

Reviewed-By: Fedor Indutny <fedor@indutny.com>
Docs fix

Reviewed-By: Fedor Indutny <fedor@indutny.com>
http_parser: Follow RFC-7230 Sec 3.2.4

RFC-7230 Sec 3.2.4 expressly forbids line-folding in header
field-names.

This change no longer allows obsolete line-folding between the
header field-name and the colon. If HTTP_PARSER_STRICT is unset,
the parser still allows space characters.

Reviewed-By: Fedor Indutny <fedor@indutny.com>