Docs: Release Notes update for 4.0

Docs: Release Notes update for 4.0

* Document C++11 requirement for compilers.

* Document ICAPS support

* Document Elliptic Curve Diffi-Hellman updates

* Document SMP updates affecting feature availability

Also, some squid.conf.documented updates for new features:

* Update icap_service TLS options to replace "ssl" prefix with "tls-"
  on newly added options.

* Remove icap_service 'sslcapath=' option from public display.
 It is still supported, but not very portable outside OpenSSL so
 do not encourage use at this time.