Update descriptions of XMPP stanza and data metrics

Affected metrics:
- xmpp_element_size_in and xmpp_element_out, which replaced
  the 'element' and 'size' metrics.
- c2s_auth_failed, s2s_auth_failed, component_auth_failed

Other changes in the docs:
- Dropped distintion between host-type-specific and global metrics,
  because it was difficult to group metrics by functionality,
  and most metrics were decribed with labels anyway.
- Merged descriptions of data metrics (tcp_data_in, tcp_data_out,
  tls_data_in, tls_data_out) for consistency and less verbosity.
- Added a new column with labels where labels differ between rows.

Author: chrzaszcz