[api] Clarifying the number range of patids

schemas/PatIdentity.yaml

@@ -190,6 +190,10 @@ patid:
     The primary purpose of this field is to show it to medical professionals working at this practice
     who can use it to search for this dossier and is a way to communicate without sharing privacy-sensitive
     details.
+    
+    The number is guaranteed to be positive, and have at most 14 digits. We suggest using a 64-bit numeric
+    type to store the number; transferring this number via IEEE-double math (common, in JSON) is safe, as
+    the number will not exceed the range within which all whole integers are perfectly representable (~`2^52`).
   type: number
   example: 1234
 pat_unid: