intro(3): Update the list of included libraries

• Extend the list of main libraries of section 3
• Extend the library functions that are included in the libc

MFC after: 2 weeks
Submitted by: Naga Chaitanya Vellanki <pnagato at protonmail dot com>
Approved by: gbe
Differential Revision: https://reviews.freebsd.org/D26476