Correct tooltip wording for watch-only wallets

[hernanmarino]

This is a continuation of https://github.com/bitcoin-core/gui/pull/37 This is a simple change, but imho necessary because it leads to confusion. The objective of this change is to provide a better description for the tooltip for the "Available Balance" balance because the current version on master mentions the word "You current spendable balance" which is a bit misleading, because that balance is not always spendable , or not always yours on watch only wallets.