subcriber: update docs for EnvFilter Builder (#2782)

The `from_env` and `try_from_env` methods on the builder had the same documentation. This change updates their docs to correctly describe their difference in behavior.


## Motivation

Make the docs more clear, so that users need not look at the source to understand the difference between these two functions.

## Solution

Updated the docs

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

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

@@ -170,15 +170,16 @@ impl Builder {
         self.parse_lossy(var)
     }
 
-    /// Returns a new [`EnvFilter`] from the directives in the in the configured
-    /// environment variable, or an error if the environment variable is not set
-    /// or contains invalid directives.
+    /// Returns a new [`EnvFilter`] from the directives in the configured
+    /// environment variable. If the environment variable is unset, no directive is added.
+    ///
+    /// An error is returned if the environment contains invalid directives.
     pub fn from_env(&self) -> Result<EnvFilter, FromEnvError> {
         let var = env::var(self.env_var_name()).unwrap_or_default();
         self.parse(var).map_err(Into::into)
     }
 
-    /// Returns a new [`EnvFilter`] from the directives in the in the configured
+    /// Returns a new [`EnvFilter`] from the directives in the configured
     /// environment variable, or an error if the environment variable is not set
     /// or contains invalid directives.
     pub fn try_from_env(&self) -> Result<EnvFilter, FromEnvError> {