Simplify primary server name section of HTTP semconv server.address

[trask]

from @lmolkova's #388 (comment):

change "The primary server name of the matched virtual host" to something like "Explicit user configuration (if provided) or a primary server name"

and @Oberon00's #388 (comment):

I agree that "virtual host" verbiage could/should probably be removed and replaced by something simpler.

[trask]

Explicit user configuration (if provided) or a primary server name

are you thinking instrumentation configuration? do we need to mention that? or is this important enough to include something like "instrumentation SHOULD provide configuration"