Missing function documentation for pcntl_wifcontinued and pcntl_forkx

[AllenJB]

Related to #2163

As I don't have a Solaris box, the pcntl_forkx documentation is based on the php-src code and comments, combined with the Solaris documentation

pcntl_wifcontinued is wrapped in an ifdef in php-src. Based on commit / NEWS history this was because it wasn't available on NetBSD (but based on search results appears to be in more recent versions). Is this significant enough to note? eg. A note along the lines of:

This function may not be available on some systems. Check you systems fork(2) man page for availability.