src.conf(5): Suggest using ccache4

We have separate ports for Ccache 3 and 4. Suggest both, rather than
only the Ccache 3 port. Rearrange the text somewhat to avoid an
excessively ragged edge on a standard 80-column terminal.

MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D58005

(cherry picked from commit a2545f68f64b4cfcc13ca2d372c8b9fef1c6331e)