grep(1): Add more EXAMPLES

• Add more EXAMPLES covering flags: -A, -B, -c, -f, -i, -H, -l, -q, -R, -w
• While here, change existing wording to use the imperative (remove "To find")
• Reword first example to be consistent with how grep(1) understand words (-w)

Approved by: manpages (bcr@)
Differential Revision: https://reviews.freebsd.org/D27264