Improve validation of recovery_target_xid GUC values.

Improve validation of recovery_target_xid GUC values.

Previously, the recovery_target_xid GUC values were not sufficiently validated.
As a result, clearly invalid inputs such as the string "bogus", a decimal value
like "1.1", or 0 (a transaction ID smaller than the minimum valid value of 3)
were unexpectedly accepted. In these cases, the value was interpreted as
transaction ID 0, which could cause recovery to behave unexpectedly.

This commit improves validation of recovery_target_xid GUC so that invalid
values are rejected with an error. This prevents recovery from proceeding
with misconfigured recovery_target_xid settings.

Also this commit updates the documentation to clarify the allowed values
for recovery_target_xid GUC.

Author: David Steele <david@pgbackrest.org>
Reviewed-by: Hüseyin Demir <huseyin.d3r@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/f14463ab-990b-4ae9-a177-998d2677aae0@pgbackrest.org