Add manual pages to the tools that were missing it.

Created by: GedRap

Added manual page to the following, which should close the issue #19 (closed):

• cp
• env
• mv
• pwd
• rmdir
• rm
• seq
• yes

All these tools print the man page when called with -h or --help operand.

It probably makes sense to squash the commits, committed separately just to make it easier to read.