rust: str: update `c_str!` documentation

rust: str: update `c_str!` documentation

Now that all literals are C-Strings, update the documentation to explain
that use of this macro should be limited to non-literal strings.

Link: https://github.com/Rust-for-Linux/linux/issues/1075
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260309-cstr-rename-macro-v2-1-25f7de75944e@kernel.org
[ Apply sentence case to comment. Reword title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>