docs: add stdlib env::var(_os) panic

src/libstd/env.rs

@@ -182,6 +182,12 @@ impl fmt::Debug for VarsOs {
 /// * Environment variable is not present
 /// * Environment variable is not valid unicode
 ///
+/// # Panics
+///
+/// This function may panic if `key` is empty, contains an ASCII equals sign
+/// `'='` or the NUL character `'\0'`, or when the value contains the NUL
+/// character.
+///
 /// # Examples
 ///
 /// ```
@@ -210,6 +216,12 @@ fn _var(key: &OsStr) -> Result<String, VarError> {
 ///
 /// [`None`]: ../option/enum.Option.html#variant.None
 ///
+/// # Panics
+///
+/// This function may panic if `key` is empty, contains an ASCII equals sign
+/// `'='` or the NUL character `'\0'`, or when the value contains the NUL
+/// character.
+///
 /// # Examples
 ///
 /// ```