Documentation updates needed for placement rules

[xhebox]

Enhancement

Sub task of #18030

1. The main page of the user doc, did not talk about show placement. So some users does not know about show placement at all. Three people has asked this question.
2. We did not point out clearly that ALTER PLACEMENT POLICY is full override for the existing policies in the user doc. Two people has asked this question and try ALTER PLACEMENT POLICY p FOLLOWERS=2 to adjust the replication numbers. I think it can be added as an counter-example.
3. PENDING scheduling state for show placement, for both rfc and user docs.
4. remove direct placement for both rfc and user docs.

[xhebox]

I would like to change the user document, but our assignment is not clear @morgo . Let us make it clear.

But whatever I could the chinese changes to user documents, since translators did not understand placement policies well, based on their previous translation of the user documents.