Add documentation for buffer types (#5410)

* Add documentation for buffer types

* address comments

* Update doc for LoadxAligned functions

Update the doc for all Load{2,3,4}Aligned and LoadxAligned<T> functions of
buffer type. We assume that those aligned version of Load{2,3,4} and
Load<T> will treat the whole buffer as type of unit{2,3,4} or T,
so the address must be aligned to size of the loaded type.

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Co-authored-by: Yong He <yonghe@outlook.com>

Author: kaizhangNV