add situation where var_os returns None

Re-introduced some of the former errors as situations where `None` is returned.

library/std/src/env.rs

@@ -236,7 +236,13 @@ fn _var(key: &OsStr) -> Result<String, VarError> {
 }
 
 /// Fetches the environment variable `key` from the current process, returning
-/// [`None`] if the variable isn't set or there's another error.
+/// [`None`] in the following situations:
+///
+///  - the environment variable isn't set
+///  - the environment variable's name contains
+/// the equal sign character (`=`) or the NUL character
+///
+/// If this is not desired, consider using [`var_os`].
 ///
 /// Note that the method will not check if the environment variable
 /// is valid Unicode. If you want to have an error on invalid UTF-8,