Clarify the purpose of interface groups in ifconfig(8)

[felix.the.red_gmail.com]

Summary

sbin/ifconfig/ifconfig.8: Clarify the use of interface groups.
sbin/ifconfig/ifconfig.8: Clean up some (not all) linter errors.

From the original PR:
"For the ifconfig(8) manual page, one of the sections mentions 'group group-name'.

• So what is a group actually? What it does or doesn't do to those interfaces? Is it simply cosmetic?
• When I need to (or not) assign something to group?
• Are there security implications with group?
• Are there performance implications with group?"

PR:231575

[zlei]

sbin/ifconfig/ifconfig.8

545

IIRC, this feature was ported from OpenBSD for pf egress interface group. It is certainly perfect valid for other use cases. End users are more creative, so I'd like to de-emphasize the current usage.

I'm not native speaker. The change is just FYI :)

[kp]

sbin/ifconfig/ifconfig.8

545

For maximum fun we don't actually create the egress group.

There are a number of implementation issues with interface groups as well (short version: the locking is absent and groups and interfaces can have the same name), but those probably don't need to be documented in the man page.

I also prefer 'can be' over 'are' here.