Fix documentation of GnuTLSCacheTimeout
Fix documentation of GnuTLSCacheTimeout

GnuTLSCacheTimeout has nothing to do with the lifetime of TLS session
tickets, but it is used for OCSP responses if they do not contain a
nextUpdate field.

This commit also includes some minor rephrasing of other parts of the
session cache documentation.

airtower-luna/mod_gnutls,airtower-luna/mod_gnutls,airtower-luna/mod_gnutls