doc: remove the document of the http disk usage api

[jackysp]

What problem does this PR solve?

The http disk usage api is not accurate, it will confuse the user sometimes.

What is changed and how it works?

Remove the introduction of this api in the document. And make some formats.

Check List

Tests

• No code

Related changes

• Need to cherry-pick to the release branch

[codecov-io]

Codecov Report

Merging #9329 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9329      +/-   ##
==========================================
+ Coverage   67.21%   67.21%   +<.01%     
==========================================
  Files         371      371              
  Lines       77552    77552              
==========================================
+ Hits        52125    52129       +4     
+ Misses      20772    20770       -2     
+ Partials     4655     4653       -2

Impacted Files	Coverage Δ
util/systimemon/systime_mon.go	80% <0%> (-20%)	⬇️
executor/executor.go	66.89% <0%> (-0.14%)	⬇️
executor/join.go	78.38% <0%> (ø)	⬆️
ddl/delete_range.go	79.36% <0%> (+4.23%)	⬆️

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[zz-jason]

LGTM

[lysu]

LGTM

[shenli]

LGTM