subscriber: fix missing doc(cfg(...)) attributes for EnvFilter (#1544)

## Motivation

Currently, some types in the `filter` module that require the
"env-filter" feature flag don't properly show in the docs that they
require this feature. This is because the `doc(cfg(feature = "env-filter"))`
attribute is only placed on the _re-export_ from the
`env` module (and on the `EnvFilter` type), not on the individual
types that are re-exported. However, placing a `doc(cfg(...))` attribute
on a re-export doesn't actually do anything.

## Solution

This commit fixes that.

tracing-subscriber/src/filter/env/directive.rs

@@ -8,6 +8,7 @@ use tracing_core::{span, Level, Metadata};
 /// A single filtering directive.
 // TODO(eliza): add a builder for programmatically constructing directives?
 #[derive(Debug, Eq, PartialEq)]
+#[cfg_attr(docsrs, doc(cfg(feature = "env-filter")))]
 pub struct Directive {
     in_span: Option<String>,
     fields: FilterVec<field::Match>,

tracing-subscriber/src/filter/env/field.rs

@@ -115,6 +115,7 @@ pub(crate) struct MatchPattern {
 
 /// Indicates that a field name specified in a filter directive was invalid.
 #[derive(Clone, Debug)]
+#[cfg_attr(docsrs, doc(cfg(feature = "env-filter")))]
 pub struct BadName {
     name: String,
 }

tracing-subscriber/src/filter/env/mod.rs

@@ -119,6 +119,7 @@ type FilterVec<T> = Vec<T>;
 
 /// Indicates that an error occurred while parsing a `EnvFilter` from an
 /// environment variable.
+#[cfg_attr(docsrs, doc(cfg(feature = "env-filter")))]
 #[derive(Debug)]
 pub struct FromEnvError {
     kind: ErrorKind,