doc: std::env::var: Returns None for names with '=' or NUL byte

The documentation incorrectly stated that std::env::var could return
an error for variable names containing '=' or the NUL byte. Copy the
correct documentation from var_os.

var_os was fixed in Commit 8a7a665, Pull Request rust-lang#109894, which
closed Issue rust-lang#109893.

This documentation was incorrectly added in commit f2c0f29, which
replaced a panic in var_os by returning None, but documented the
change as "May error if ...".

Reference the specific error values and link to them.

std/src/env.rs

@@ -198,13 +198,16 @@ impl fmt::Debug for VarsOs {
 ///
 /// # Errors
 ///
-/// This function will return an error if the environment variable isn't set.
+/// This function returns [`VarError::NotPresent`] if the environment variable
+/// isn't set.
 ///
-/// This function may return an error if the environment variable's name contains
-/// the equal sign character (`=`) or the NUL character.
+/// This function may return [`VarError::NotPresent`] if the
+/// environment variable's name contains the equal sign character (`=`) or the
+/// NUL character.
 ///
-/// This function will return an error if the environment variable's value is
-/// not valid Unicode. If this is not desired, consider using [`var_os`].
+/// This function will return [`VarError::NotUnicode`] if the environment
+/// variable's value is not valid Unicode. If this is not desired, consider
+/// using [`var_os`].
 ///
 /// # Examples
 ///