doc: Fix description of gix_testtools::Env::unset

The `unset` method inadvertently had the same docstring as `set`,
even though this was not correct for `unset`. This fixes that, and
also rewords the `Env` docstring to better account for the ability
to unset.

Author: EliahKagan

tests/tools/src/lib.rs

@@ -824,7 +824,7 @@ fn family_name() -> &'static str {
     }
 }
 
-/// A utility to set environment variables, while unsetting them (or resetting them to their previous value) on drop.
+/// A utility to set and unset environment variables, while restoring or removing them on drop.
 #[derive(Default)]
 pub struct Env<'a> {
     altered_vars: Vec<(&'a str, Option<OsString>)>,
@@ -846,7 +846,7 @@ impl<'a> Env<'a> {
         self
     }
 
-    /// Set `var` to `value`.
+    /// Unset `var`.
     pub fn unset(mut self, var: &'a str) -> Self {
         let prev = std::env::var_os(var);
         std::env::remove_var(var);