docs: Raise visibility of 'string' feature

src/_features.rs

@@ -19,7 +19,7 @@
 //! * **env**: Turns on the usage of environment variables during parsing.
 //! * **unicode**: Turns on support for unicode characters (including emoji) in arguments and help messages.
 //! * **wrap_help**: Turns on the help text wrapping feature, based on the terminal size.
-//! * **string**: Allow runtime generated strings
+//! * **string**: Allow runtime generated strings (e.g. with [`Str`][crate::builder::Str]).
 //!
 //! #### Experimental features
 //!

src/builder/os_str.rs

@@ -1,6 +1,9 @@
 use crate::builder::Str;
 
 /// A UTF-8-encoded fixed string
+///
+/// **NOTE:** To support dynamic values (i.e. `OsString`), enable the [`string`
+/// feature][crate::_features]
 #[derive(Default, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
 pub struct OsStr {
     name: Inner,

src/builder/str.rs

@@ -1,4 +1,7 @@
 /// A UTF-8-encoded fixed string
+///
+/// **NOTE:** To support dynamic values (i.e. `String`), enable the [`string`
+/// feature][crate::_features]
 #[derive(Default, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
 pub struct Str {
     name: Inner,