docs(api/result): clarify that result.rowCount can be null

[futile]

I stumbled upon this when running a LOCK-command and looking at the result.rowCount-field, which was null.

This is because result.rowCount is initialized to null, but only set to an int-value if the returned command tag consists of more than one word, which is not the case in general.

The postgres docs (https://www.postgresql.org/docs/current/protocol-message-formats.html, ctrl+f for CommandComplete) say:

The command tag. This is usually a single word that identifies which SQL command was completed.

This explains why, for commands such as LOCK, there is no [ROWS]-field to parse.

See the logic in node-postgres here:

node-postgres/packages/pg/lib/result.js

Lines 13 to 47 in b357e18

	this.rowCount = null
	this.oid = null
	this.rows = []
	this.fields = []
	this._parsers = undefined
	this._types = types
	this.RowCtor = null
	this.rowAsArray = rowMode === 'array'
	if (this.rowAsArray) {
	this.parseRow = this._parseRowAsArray
	}
	}
	// adds a command complete message
	addCommandComplete(msg) {
	var match
	if (msg.text) {
	// pure javascript
	match = matchRegexp.exec(msg.text)
	} else {
	// native bindings
	match = matchRegexp.exec(msg.command)
	}
	if (match) {
	this.command = match[1]
	if (match[3]) {
	// COMMMAND OID ROWS
	this.oid = parseInt(match[2], 10)
	this.rowCount = parseInt(match[3], 10)
	} else if (match[2]) {
	// COMMAND ROWS
	this.rowCount = parseInt(match[2], 10)
	}
	}
	}

Running a command such as LOCK, postgres will return a command tag of simply the form LOCK, and thus result.rowCount will stay null.

This PR clarififes this in the docs.

[brianc]

ohhh nice doc addition! 👏 🙏