[abp002]

Adds the missing Errors sections and changelog rows on the gettext pages listed in #6001.

• dcgettext() and dcngettext() throw if domain is empty or longer than 1024 bytes, if a message is longer than 4096 bytes, or if category is LC_ALL. Added an 8.4.0 row for the empty domain and LC_ALL, and an 8.0.0 row for the lengths.
• gettext() and ngettext() throw if a message is longer than 4096 bytes (8.0.0).
• textdomain() also throws for a domain longer than 1024 bytes (8.0.0) and for "0" (8.4.0). Before 8.4, "" and "0" just returned the current domain.
• bindtextdomain() throws if domain is empty or longer than 1024 bytes (8.0.0, it used to warn and return false). Also noted that "0" means the current directory like "", and that a directory that doesn't exist returns false.
• bind_textdomain_codeset() also throws for a domain longer than 1024 bytes (8.0.0), and the return value now says it returns false when no encoding is set.

Checked against ext/gettext/gettext.c on 7.4, 8.0, 8.3, 8.4 and 8.5, and ran each case on those versions.

Fixes #6001