`WebUtility.UrlDecode` does not document conversion of '+' into ' '

[matt-phylum]

The documentation for UrlEncode was updated in #3124 to indicate that ' ' is converted into '+' on encode.

However:

• The documentation for UrlEncodeToBytes needs the same update.
• The documentation for UrlDecode and UrlDecodeToBytes needs a similar update to indicate that '+' is converted into ' ' on decode.
• All of these pages give the example that < and > are encoded as %3c and %3e, but this is incorrect. < and > are encoded as %3C and %3E. RFC 3986 says "For consistency, URI producers and normalizers should use uppercase hexadecimal digits for all percent-encodings," and that's what the implementation does.