Updated manual pages to address situations where multiple usernames

resolve to the same UID and how the "as" syntax in the doas.conf
file is affected.

doas.1

@@ -88,6 +88,9 @@ or
 Execute the command as
 .Ar user .
 The default is root.
+Please note: On some systems multiple usernames can resolve to one UID. For example, root and
+toor both resolve to UID 0 on FreeBSD. Please see the "as" syntax section of the doas.conf manual page for details on
+how doas handles this situation.
 .It Fl -
 Any dashes after a combined double dash (--) will be interpreted as part of the command to be run or its parameters. Not an argument passed to doas itself.
 .El

doas.conf.5

@@ -91,6 +91,18 @@ Numeric IDs are also accepted.
 .It Ic as Ar target
 The target user the running user is allowed to run the command as.
 The default is all users.
+Please be aware that on some systems multiple usernames can resolve to
+the same UID. For example, on FreeBSD it is common to have both toor and root
+resolve to UID 0. The doas utility will allow any username that matches a shared
+UID to match when the -u flag is invoked. This means if you have the rule 
+.Bd -literal -offset indent
+permit alice as toor 
+
+.Ed
+in your doas.conf file, the command "doas -u root" can be successfully run from
+Alice's account in situations where both root and toor share the same UID. In short,
+doas cares about the UID behind the username, not the specific username given on the
+command line.
 .It Ic cmd Ar command
 The command the user is allowed or denied to run.
 The default is all commands.