[Documentation] Add mnemonics for r and a targets

[souvlakeeb]

In the docs you say:

The targets b, B, r, and a are aliases for ), }, ], and > (the first two mirror Vim; the second two are completely arbitrary and subject to change).

I would argue that the last two are not arbitrary at all, and instead, excellent mnemonics! Considering that the first two mirror Vim (:h text-objects), this would be the completed table:

Target	Alias	Mnemonic
b	)	b(lock)
B	}	B(lock)
r	]	r(ectangle bracket)
a	>	a(ngle bracket)

Technically, it's a square bracket. I like the mnemonic less, but I leave it to you:

Target	Alias	Mnemonic
r	]	(squa)r(e bracket)