Update UserGuide "Mesh Support" section

[pp-mo]

Various "Summer of Mesh" changes are affecting this

• deprecation of PARSE_UGRID_ON_LOAD Enable UGRID loading always; deprecate PARSE_UGRID_ON_LOAD. #6054
• move+rename of iris.experimental.ugrid API Mesh nonexperimental #6061 + its follow-ons

Some changes have happened automatically, and specific failures will be fixed.
But it's probably worth re-reviewing all the advice before release.

[pp-mo]

Done in #6077