update documentation
Rename gnus-summary-ext-limit-expression to gnus-summary-ext-limit-filter

Also remove old version of gnus-summary-ext-limit-filter, and add saved
filters to read history for gnus-summary-ext-read-limit-expression

Altered items: gnus-summary-ext-saved-filters,
gnus-summary-ext-read-limit-expression, gnus-summary-ext-limit-filter
Remove: gnus-summary-ext-read-limit-expression

Altered items: gnus-summary-ext-read-limit-expression,
gnus-summary-ext-limit-filter