wpi: refresh manpage.

Reflect all recent changes in the manpage:

• add adhoc-demo and hostap into list of supported modes;

add few examples for them;

• mention encryption/decryption offload for CCMP cipher;
• extend list of driver messages in the DIAGNOSTICS;
• document hostap mode limitations / powersave instability

in the CAVEATS section.

Differential Revision: https://reviews.freebsd.org/D5149