Improve QueryExecModeCacheDescribe and clarify documentation

QueryExecModeCacheDescribe actually is safe even when the schema or
search_path is modified. It may return an error on the first execution
but it should never silently encode or decode a value incorrectly. Add a
test to demonstrate and ensure this behavior.

Update documentation of QueryExecModeCacheDescribe to remove warning of
undetected result decoding errors.

Update documentation of QueryExecModeCacheStatement and
QueryExecModeCacheDescribe to indicate that the first execution of an
invalidated statement may fail.

internal/stmtcache/stmtcache.go

@@ -38,19 +38,3 @@ type Cache interface {
 	// Cap returns the maximum number of cached prepared statement descriptions.
 	Cap() int
 }
-
-func IsStatementInvalid(err error) bool {
-	pgErr, ok := err.(*pgconn.PgError)
-	if !ok {
-		return false
-	}
-
-	// https://github.com/jackc/pgx/issues/1162
-	//
-	// We used to look for the message "cached plan must not change result type". However, that message can be localized.
-	// Unfortunately, error code "0A000" - "FEATURE NOT SUPPORTED" is used for many different errors and the only way to
-	// tell the difference is by the message. But all that happens is we clear a statement that we otherwise wouldn't
-	// have so it should be safe.
-	possibleInvalidCachedPlanError := pgErr.Code == "0A000"
-	return possibleInvalidCachedPlanError
-}