dlopen.3: Clarify when an executable's symbols can be used for symbol resolution

dlopen.3: Clarify when an executable's symbols can be used for symbol resolution

The --export-dynamic linker option is not the only way that main's
global symbols may end up in the dynamic symbol table and thus be
used to satisfy symbol reference in a shared object. A symbol
may also be placed into the dynamic symbol table if ld(1)
notices a dependency in another object during the static link.

Verified by experiment; see previous commit.

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