Clarify behavior of \tl_if_novalue:n(TF) (see #1402)

l3kernel/CHANGELOG.md

@@ -14,6 +14,7 @@ this project uses date-based 'snapshot' version identifiers.
 
 ### Changed
 - Improved method to suppress `l3bitset` where required
+- Clarify behavior of `\tl_of_novalue:n(TF)` (see issue \#1402)
 
 ### Deprecated
 - `\tl_build_get:NN` in favor of `\tl_build_get_intermediate:NN`

l3kernel/l3tl.dtx

@@ -330,8 +330,12 @@
 %     \cs{tl_if_novalue_p:n} \Arg{token list}
 %     \cs{tl_if_novalue:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
 %   \end{syntax}
-%   Tests if the \meta{token list} is exactly equal to the special
-%   \cs{c_novalue_tl} marker. This function is intended to allow construction
+%   Tests if the \meta{token list} is exactly equal to the content of special
+%   \cs{c_novalue_tl} marker, hence it is functionally the same as
+%   \begin{quote}
+%     \cs{tl_if_eq:NnTF} \cs{c_novalue_tl} \Arg{token list} \Arg{true code} \Arg{false code}
+%   \end{quote}
+%   but expandable. This function is intended to allow construction
 %   of flexible document interface structures in which missing optional
 %   arguments are detected.
 % \end{function}