refactor: update documents and rename variables

[zhaoyongjie]

SUMMARY

This PR address some comments from the previous PR and PR.

• rename the xAxis to the xAxisLabel
• add more description for the time grain migration

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #22074 (6057e33) into master (4496748) will not change coverage.
The diff coverage is 90.00%.

@@           Coverage Diff           @@
##           master   #22074   +/-   ##
=======================================
  Coverage   67.12%   67.12%           
=======================================
  Files        1818     1818           
  Lines       69639    69639           
  Branches     7496     7496           
=======================================
  Hits        46747    46747           
  Misses      20954    20954           
  Partials     1938     1938           

Flag	Coverage Δ
javascript	53.70% <90.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...BigNumber/BigNumberWithTrendline/transformProps.ts	47.05% <66.66%> (ø)
...t-ui-chart-controls/src/operators/pivotOperator.ts	100.00% <100.00%> (ø)
...ui-chart-controls/src/operators/prophetOperator.ts	100.00% <100.00%> (ø)
...-ui-chart-controls/src/operators/renameOperator.ts	100.00% <100.00%> (ø)
...controls/src/operators/timeComparePivotOperator.ts	100.00% <100.00%> (ø)

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[stephenLYZ]

LGTM

[villebro]

LGTM, thanks for the clarifications here!