The actual results (shown below) are different than shown on this doc page. The reason is because the second parameter to the UDF that is passed to SPI_exec is the maximum number of rows to return, or 0 for no limit. It is not the maximum number of rows to process. In the case of "SELECT execq('INSERT INTO a SELECT x + 2 FROM a', 1)", it returned 0 rows, but it inserted (processed) 2 rows. This example should be corrected.
db=# SELECT execq('INSERT INTO a SELECT x + 2 FROM a', 1); execq ------- 2 (1 row)
SPI_exec sees "INSERT 0 2" as the command tag from the SQL command you passed and so 2 is the output of the execq function call.
No INFO messages appear because you did not include a returning clause. The 1 you passed to the call is immaterial if the query you supply doesn't produce a result set.