[Sync-EN] Fix the pcntl reference pages and document the 8.4 failure return

[lacatoire]

Syncs seven reference/pcntl/ files with php/doc-en#5767.

• pcntl_exec(): the return value said only "returns false". It now says false is returned on failure and that on success the function does not return at all, the process image being replaced.
• pcntl_unshare(): returns true/false, not 0/-1.
• pcntl_sigprocmask(), pcntl_sigtimedwait(), pcntl_sigwaitinfo(): the changelog rows named a signal parameter that does not exist, the parameter is signals; the "not an int" row said ValueError where it is a TypeError; pcntl_sigprocmask() gains the SIG_SETMASK exemption for an empty array; pcntl_sigtimedwait() states nanoseconds must be between 0 and 1e9, and that the both-zero row is about both being 0, not "less than 0". pcntl_sigtimedwait() and pcntl_sigwaitinfo() gain the row about false being returned on failure where -1 was returned in some cases.
• constants.xml: the SI_MSGGQ entry, which had an empty description, is removed as upstream.
• versions.xml gains the missing pcntl_forkx, pcntl_getcpu, pcntl_setns and pcntl_wifcontinued entries.
• ValueError, TypeError and friends are marked up as exceptionname, and the four see-also simplelists lose their para wrapper.

EN-Revision bumped to 253f4bdd29fc3938030dce22882611ceefa9f74a on the seven files.

Fixes: #1685