curl_getdate.md: document two-digit year handling

Mentioned-by: Paul Gilmartin
Ref: https://curl.se/mail/archive-2024-04/0014.html
Closes #13494

docs/libcurl/curl_getdate.md

@@ -47,6 +47,9 @@ Can be specified several ways. Month names can only be three-letter English
 abbreviations, numbers can be zero-prefixed and the year may use 2 or 4
 digits. Examples: 06 Nov 1994, 06-Nov-94 and Nov-94 6.
 
+If the year appears to be below 100 (two-digit), any year after 70 is assumed
+to be 1900 + the given year. All others are 2000 + the given year.
+
 ## time of the day items
 
 This string specifies the time on a given day. You must specify it with 6