Rollup merge of #54860 - mandeep:vec-initialize, r=alexcrichton
Rollup merge of #54860 - mandeep:vec-initialize, r=alexcrichton

Add doc comments about safest way to initialize a vector of zeros

This adds more information about the vec! macro as discussed in #54628. I think this is a good starting point, but I think additional detail is needed so that we can explain why vec! is safer than the alternatives.

GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust