doc: add HOWTO document about Documenting public Functions and Macros

doc: add HOWTO document about Documenting public Functions and Macros

Co-authored-by: Matthias St. Pierre <matthias.st.pierre@ncp-e.com>
Co-authored-by: Tomáš Mráz <tm@t8m.info>
Co-authored-by: Lin2Jing4 <35728132+Lin2Jing4@users.noreply.github.com>
Co-authored-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17926)