Improve descriptions of LocalVector, recommend it when appropriate

[beicause]

The current documentation doesn't highlight the advantages of LocalVector and may lead to misunderstandings and discourage its use.
In fact I rarely noticed the differences between LocalVector and Vector before, until seeing many PRs in 4.5 replacing Vector with LocalVector. I think we should provide a clearer explanation of LocalVector in the documentation.

[smix8]

The main benefit of LocalVector, apart from no copy-on-write, is that a LocalVector keeps its allocated memory. So when you frequently edit and clear stuff and it is used internally only you want to use a LocalVector to not waste performance on all the frequent memory alloc.

[beicause]

The main benefit of LocalVector, apart from no copy-on-write, is that a LocalVector keeps its allocated memory. So when you frequently edit and clear stuff and it is used internally only you want to use a LocalVector to not waste performance on all the frequent memory alloc.

Will this be changed? Related: godotengine/godot#105928 (review)