tracing: check log max level prior to dispatch check

[hawkw]

Currently, when the log feature is enabled but the log-always
feature is not, we check the (dynamic, but inexpensive)
dispatch::has_been_set variable before checking if the event's
level would be enabled by log's static max level features.

Although this check is not terribly costly in terms of performance, it
does result in code being generated even when the log crate disables
some levels statically. This means that we will always generate some
code when the log feature is disabled. This isn't ideal --- the static
max level features should result in us generating no code whatsoever
for disabled trace points.

This commit moves the static max level check outside of the dispatch
check. Now, we should generate 0 bytes of assembly when both log and
tracing disable an event with their static filtering features.

I've also updated the docs to explain the relationship between tracing
and log's separate static filtering.

Fixes #1174

[bobrik]

This fails, similar to #1191:

error[E0433]: failed to resolve: maybe a missing crate `dispatcher`?
    --> /Users/ivan/projects/tracing/tracing/src/macros.rs:2352:25
     |
2352 |               if !$crate::dispatcher::has_been_set() {
     |                           ^^^^^^^^^^ maybe a missing crate `dispatcher`?
     |
    ::: /Users/ivan/projects/tracing/tracing/src/span.rs:765:9
     |
765  | /         if_log_enabled! { crate::Level::TRACE, {
766  | |             if let Some(ref meta) = self.meta {
767  | |                 self.log(ACTIVITY_LOG_TARGET, log::Level::Trace, format_args!("-> {}", meta.name()));
768  | |             }
769  | |         }}
     | |__________- in this macro invocation

[bobrik]

I opened #1217 to address this.