Add SSL_get_rfd and SSL_get_wfd. Various documentation updates.

[kareem-wolfssl]

Description

Fixes #9517, #6008, #9084, #5602, wolfSSL/wolfssl-nginx#25

Testing

Built in tests

Checklist

• added tests
• updated/added doxygen
• updated appropriate READMEs
• Updated manual and documentation

[devin-ai-integration]

🛟 Devin Lifeguard found 1 likely issues in this PR

• no-void-functions snippet snippet: Change both new APIs to return an error code (e.g., int) and have every call-site handle the return value, e.g., int wolfSSL_CTX_set_default_passwd_cb(...); and int wolfSSL_CTX_set_default_passwd_cb_userdata(...);.

@kareem-wolfssl
please take a look at the above issues which Devin flagged. Devin will not fix these issues automatically.