Editorial pass and consistency updates

[ekinnear]

You can view just these changes at 95b51df:

• Fix some typos, grammar, whitespace, etc.

You can view just these changes at 9a81443:

• "A client MUST NOT send certificates to the server" vs. "The server SHOULD close the connection upon receipt" --> consistently use MUST for both endpoints
• "A server sends a SERVER_CERTIFICATE frame on stream 0 for HTTP/2 and on the control stream for HTTP/3" --> drop the duplicate description of what's actually specified below now
• "SERVER_CERTIFICATE_UNREADABLE" was defined but never used, update the text above to use it
• "SERVER_CERTIFICATE_UNREADABLE" was also not in the IANA considerations section, add it

[ekinnear]

Added a few more in d3e0036:

• "Upon receiving a complete series of SERVER_CERTIFICATE frames" --> just say "frame"
• "this SHOULD be treated as a connection error" --> MUST and be specific about the error code

[tfpauly]

Thanks, looks good!

[LPardue]

Lgtm, thanks!