…1162)

Closes #1161

Adds parameter tables and formal API reference sections to three pages
that previously had examples but no parameter documentation.

pushgateway.md: documents push_to_gateway, pushadd_to_gateway,
delete_from_gateway, instance_ip_grouping_key, and all four built-in
handlers (default, basic_auth, tls_auth, passthrough_redirect).

textfile.md: documents write_to_textfile with all four parameters
including the previously undocumented escaping and tmpdir, plus atomic
write semantics and error behavior.

multiprocess/_index.md: documents MultiProcessCollector constructor and
mark_process_dead with parameter tables including the previously
undocumented path parameter on both.

Signed-off-by: k1chik <107162115+k1chik@users.noreply.github.com>