Document the percent-encoding mechanism

doc/manual/src/release-notes/rl-next.md

@@ -1 +1,5 @@
 # Release X.Y (202?-??-??)
+
+- [URL flake references](@docroot@/command-ref/new-cli/nix3-flake.md#flake-references) now support [percent-encoded](https://datatracker.ietf.org/doc/html/rfc3986#section-2.1) characters.
+
+- [Path-like flake references](@docroot@/command-ref/new-cli/nix3-flake.md#path-like-syntax) now accept arbitrary unicode characters (except `#` and `?`).

src/nix/flake.md

@@ -67,6 +67,13 @@ inputs.nixpkgs = {
 };
 ```
 
+Following [RFC 3986](https://datatracker.ietf.org/doc/html/rfc3986#section-2.1),
+characters outside of the allowed range (i.e. are neither [reserved
+character](https://datatracker.ietf.org/doc/html/rfc3986#section-2.2) or
+[unreserved
+characters](https://datatracker.ietf.org/doc/html/rfc3986#section-2.3)) must be
+percent-encoded.
+
 ### Examples
 
 Here are some examples of flake references in their URL-like representation:
@@ -103,10 +110,15 @@ The semantic of such a path is as follows:
     2. The filesystem root (/), or
     3. A folder on a different mount point.
 
+Contrary to URL-like reference, path-like flake references can contain
+arbitrary unicode characters (except `#` and `?`).
+
 ### Examples
 
 * `.`: The flake to which the current directory belongs to.
 * `/home/alice/src/patchelf`: A flake in some other directory.
+* `./../sub directory/with Ûñî©ôδ€`: A flake in another relative directory that
+  has Unicode characters in its name.
 
 ## Flake reference attributes