Doc: remove obsolete advice about manually inserting snprintf into build.

Doc: remove obsolete advice about manually inserting snprintf into build.

This para is obsolete, first because nobody is using Solaris 7 anymore,
and second because if someone was, configure should catch the snprintf
buffer overrun problem automatically (since commit 9bed827b1), and third
because this is incorrect advice about how to manually force use of
snprintf.c anyway, and has been so at least since commit 3bc6bdf32.
The lack of complaints about it reinforces the conclusion that Solaris 7
no longer exists in the wild; so I don't feel a need to insert correct
advice instead.