Hi Oliver,
At 2021-01-14T21:37:21+0100, Oliver Corff wrote:

Filip's question prompted me to have a look at the eqn(1) manpage, and
unfortunately, the "See also" section near the end does not point to
groff_char(7) where all the symbols groff knows are listed.
What about appending this short paragraph and change it from
SEE ALSO: groff(1), troff(1), pic(1), groff_font(5), The TeXbook
to
SEE ALSO: groff_char(7), groff(1), troff(1), pic(1), groff_font(5), The
TeXbook
and perhaps include a short statement somewhere in the text body that
all math symbols are detailed in a separate manpage?

I pushed something along the lines you describe.
commit 34ecb24b95edf60d231b71ef2782c5a8994098cb
Author: G. Branden Robinson <g.branden.robinson@gmail.com>
Date: Fri Jan 15 12:24:31 2021 +1100
eqn(1): Add cross reference to groff_char(7).
...prompted by recommendation from Oliver Corff.
diff --git a/src/preproc/eqn/eqn.1.man b/src/preproc/eqn/eqn.1.man
index 2ea4ea5c..db672e28 100644
--- a/src/preproc/eqn/eqn.1.man
+++ b/src/preproc/eqn/eqn.1.man
@@ -1570,6 +1570,15 @@ Addison-Wesley Professional.
.
.
.LP
+.IR groff_char (@MAN7EXT@),
+particularly subsections \[lq]Logical symbols\[rq],
+\[lq]Mathematical symbols\[rq],
+and \[lq]Greek glyphs\[rq],
+documents a variety of special character escapes useful in mathematical
+typesetting.
+.
+.
+.LP
.IR groff (@MAN1EXT@),
.IR \%@g@troff (@MAN1EXT@),
.IR \%@g@pic (@MAN1EXT@),
Regards,
Branden