Various RT doc fixes

RT1556: doc/crypto/threads.pod
RT2024: Missing pages mentioned in crypto.pod
RT2890: Wrong size in ERR_string_error description.
RT3461: Better description of PEM Encryption
        (Jeffrey Walton <noloader@gmail.com>)
        Also, fix up formatting and removed some code examples
        that encourage unsafe patterns, like unencrypted private
        keys (Rich Salz)
RT4240: Document some speed flags (Tomas Mraz <tmraz@redhat.com>)
RT4260: Fix return value doc for X509_REQ_sign and X509_sign
        (Laetitia Baudoin <lbaudoin@google.com>)

Reviewed-by: Emilia Käsper <emilia@openssl.org>