Clarify ldap_connect/ldap_bind/ldap_set_option behavior

[lacatoire]

Summary

• Clarify that ldap_connect() does NOT open a network connection, only validates URI and
  initializes parameters
• Document that ldap_bind() is where the actual TCP connection is established
• Add note to ldap_set_option() that TLS options (LDAP_OPT_X_TLS_*) must be set globally
  (with null) before ldap_bind()
• Document the correct workflow: ldap_connect() → ldap_set_option() → ldap_bind()

Fixes #3526