docs: Clarify where to put texmf.cfg

[muzimuzhi]

I find the current doc for texmf.cfg a bit confusing.

l3build/l3build.dtx

Lines 1180 to 1190 in 2cd3dff

	% \subsection{Settings in \texttt{texmf.cnf}}
	%
	% To allow application of non-standard \TeX{} trees or similar non-standard
	% settings, \pkg{l3build} enables searching for a \texttt{texmf.cnf} file
	% by setting the envirnmental variable \texttt{TEXMFCNF}. This might
	% for example be used with a file containing
	% \begin{verbatim}
	% TEXMFAUXTREES = ../../texmf,
	% \end{verbatim}
	% for adding a local tree within the development repository (assuming the
	% typical \pkg{l3build} layout).

Clarifying that texmf.cfg can be put in

• for testing, directory testsuppdir, or directory supportdir and matches checksuppfiles
• for doc typesetting, directory supportdir and matches typesetsuppfiles

may be helpful. The directory structure and related snippet of build.lua in luaotfload could also be shown as an example.