Clarify that parameter generator does not evaluate immediately

This aims to avoid confusion as to what kinds of computations are valid inside
a parameter generator expression, calling out flags as a supported use case.

PiperOrigin-RevId: 483397027
Change-Id: I2d036fae95120d617f30a5566ea7498ce1f9bfb6

Author: Abseil Team

docs/advanced.md

@@ -1095,6 +1095,11 @@ instantiation of the test suite. The next argument is the name of the test
 pattern, and the last is the
 [parameter generator](reference/testing.md#param-generators).
 
+The parameter generator expression is not evaluated until GoogleTest is
+initialized (via `InitGoogleTest()`). Any prior initialization done in the
+`main` function will be accessible from the parameter generator, for example,
+the results of flag parsing.
+
 You can instantiate a test pattern more than once, so to distinguish different
 instances of the pattern, the instantiation name is added as a prefix to the
 actual test suite name. Remember to pick unique prefixes for different