dlsym.3: Describe a case where a symbol value may be NULL

dlsym.3: Describe a case where a symbol value may be NULL

Remove a longstanding mystery in the text of the page, by
explaining a case where the value returned for a symbol may be
NULL. (However, there are presumably other cases, since the text
in the dlsym(3) manual page pre-dates the invention of IFUNCs.)

See also
https://stackoverflow.com/questions/13941944/why-can-the-value-of-the-symbol-returned-by-dlsym-be-null

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>