mention when the function was added to the lib

docs/libcurl/curl_easy_strerror.3

@@ -12,6 +12,8 @@ curl_easy_strerror - return string describing error code
 .SH DESCRIPTION
 The curl_easy_strerror() function returns a string describing the CURLcode
 error code passed in the argument \fIerrornum\fP.
+
+This function was added in libcurl 7.12.0
 .SH RETURN VALUE
 A pointer to a zero terminated string.
 .SH "SEE ALSO"

docs/libcurl/curl_multi_strerror.3

@@ -12,6 +12,8 @@ curl_multi_strerror - return string describing error code
 .SH DESCRIPTION
 The curl_multi_strerror() function returns a string describing the CURLMcode
 error code passed in the argument \fIerrornum\fP.
+
+This function was added in libcurl 7.12.0
 .SH RETURN VALUE
 A pointer to a zero terminated string.
 .SH "SEE ALSO"

docs/libcurl/curl_share_strerror.3

@@ -12,6 +12,8 @@ curl_share_strerror - return string describing error code
 .SH DESCRIPTION
 The curl_share_strerror() function returns a string describing the CURLSHcode
 error code passed in the argument \fIerrornum\fP.
+
+This function was added in libcurl 7.12.0
 .SH RETURN VALUE
 A pointer to a zero terminated string.
 .SH "SEE ALSO"