manual.yml: revise section on identity, clarifying what is subject to change

Mostly clarifications w.r.t. numbers.

The anomalous behavior of leading minus signs is documented, with an explicit notice that this will probably change.

tests/man.test

@@ -2,9 +2,17 @@
 "Hello, world!"
 "Hello, world!"
 
-. | tojson
+.
+0.12345678901234567890123456789
+0.12345678901234567890123456789
+
+[., tojson]
 12345678909876543212345
-"12345678909876543212345"
+[12345678909876543212345,"12345678909876543212345"]
+
+. < 0.12345678901234567890123456788
+0.12345678901234567890123456789
+false
 
 map([., . == 1]) | tojson
 [1, 1.000, 1.0, 100e-2]