libcurl documentation: clarifications and typos

* Elaborates on default values of some curl_easy_setopt() options.
* Reminds the user to cast variadic arguments to curl_easy_setopt() to
  'void *' where curl internally interprets them as such.
* Clarifies the working of the CURLOPT_SEEKFUNCTION option for
  curl_easy_setopt().
* Fixes typo 'forth' → 'fourth'.
* Elaborates on CURL_SOCKET_TIMEOUT.
* Adds some missing periods.
* Notes that the return value of curl_version() must not be passed to
  free().

docs/libcurl/curl_version.3

@@ -31,6 +31,7 @@ curl_version - returns the libcurl version string
 Returns a human readable string with the version number of libcurl and some of
 its important components (like OpenSSL version).
 .SH RETURN VALUE
-A pointer to a zero terminated string.
+A pointer to a zero terminated string. The string resides in a statically
+allocated buffer and must not be freed by the caller.
 .SH "SEE ALSO"
 .BR curl_version_info "(3)"