DOC: document in more detail what a BIO_read_ex() via BIO_f_buffer() does
DOC: document in more detail what a BIO_read_ex() via BIO_f_buffer() does

The BIO_f_buffer() documentation tells in enough detail how it affects
BIO_gets(), but not how it affects BIO_read_ex().  This change
remedies that.

Fixes #10859

Reviewed-by: Tim Hudson <eb0be8e447f673b41b10eda09d63cd72526b0a42@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10890)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl