[DOCFIX] Add Allocator & Reviewer sections to Caching.md
[DOCFIX] Add Allocator & Reviewer sections to Caching.md

This change:
1. Adds Allocator as a section. This is an open interface configuring
the allocation strategy, but is not documented.
1. Adds Reviewer as a section. This is a new interface added in 2.4.1.
1. Fixes multiple typos and updates a few doc link formats.

pr-link: Alluxio/alluxio#12566
change-id: cid-c5f4106d10fb8624e5b577c244a8e8308c95ad4b
calvinjia/tachyon,bf8086/alluxio,bf8086/alluxio,maobaolong/alluxio,wwjiang007/alluxio,maobaolong/alluxio,bf8086/alluxio,calvinjia/tachyon,calvinjia/tachyon,Alluxio/alluxio,maobaolong/alluxio,maobaolong/alluxio,Alluxio/alluxio,Alluxio/alluxio,calvinjia/tachyon,wwjiang007/alluxio,wwjiang007/alluxio,maobaolong/alluxio,bf8086/alluxio,wwjiang007/alluxio,maobaolong/alluxio,bf8086/alluxio,Alluxio/alluxio,calvinjia/tachyon,bf8086/alluxio,calvinjia/tachyon,wwjiang007/alluxio,maobaolong/alluxio,Alluxio/alluxio,Alluxio/alluxio,maobaolong/alluxio,wwjiang007/alluxio,Alluxio/alluxio,maobaolong/alluxio,bf8086/alluxio,calvinjia/tachyon,maobaolong/alluxio,bf8086/alluxio,Alluxio/alluxio,wwjiang007/alluxio,Alluxio/alluxio,calvinjia/tachyon,Alluxio/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,wwjiang007/alluxio