doc: fix default maxBuffer size

Correctly document the default maxBuffer size for execSync,
execFileSync, and spawnSync. It is 200 * 1024, not Infinity.
Add tests to verify behaviour is as documented.

PR-URL: https://github.com/nodejs/node/pull/22894


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>