[DOCS] Correct geography area/length radius wording: spherical mean radius, not authalic or WGS84 spheroid

[jiayuasu]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a ticket?

• No:
  • this is a documentation update. The PR name follows the format [DOCS] my subject

What changes were proposed in this PR?

Corrects the radius wording in the geography area/length docs to match the implementation.

common/src/main/java/org/apache/sedona/common/geography/Functions.java computes ST_Area and ST_Length on a sphere, scaling by org.apache.sedona.common.sphere.Haversine.AVG_EARTH_RADIUS, which is defined as 6371008.0 meters. That value is the mean Earth radius (IUGG R1 = (2a + b) / 3 ≈ 6 371 008.8 m), and the constant is named AVG_EARTH_RADIUS accordingly. It is not the WGS84 ellipsoid, and it is not the authalic radius (R2 ≈ 6 371 007.2 m).

Two inaccuracies are fixed:

• The ST_Area overview row in docs/api/sql/geography/Geography-Functions.md said "WGS84 spheroid" — the computation is spherical, not ellipsoidal.
• Several pages labeled R = 6 371 008 m as the "authalic Earth radius" — that value is the mean radius. Reworded to "mean Earth radius" in the Spark ST_Area/ST_Length pages and the SedonaFlink geography overview and ST_Area page.

How was this patch tested?

mkdocs build succeeds; the changed pages have no new warnings and all links resolve.

Did this PR include necessary documentation updates?

• Yes, I have updated the documentation.

[Copilot]

Pull request overview

Updates Sedona geography documentation to correctly describe the Earth model and radius used by ST_Area/ST_Length, aligning docs with the spherical implementation (mean Earth radius R = 6 371 008 m) rather than WGS84/authalic wording.

Changes:

• Reword ST_Area / ST_Length docs to label R = 6 371 008 m as the mean Earth radius (not authalic).
• Correct ST_Area overview wording to reflect spherical (not WGS84 ellipsoidal) computation.
• Align SedonaFlink geography overview and ST_Area page wording with the spherical mean-radius model.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/api/sql/geography/Geography-Functions/ST_Length.md	Updates ST_Length intro to “mean Earth radius”.
docs/api/sql/geography/Geography-Functions/ST_Area.md	Updates ST_Area intro to “mean Earth radius”.
docs/api/sql/geography/Geography-Functions.md	Fixes ST_Area summary row to describe spherical mean-radius computation (not WGS84).
docs/api/flink/Geography-Functions/ST_Area.md	Updates Flink ST_Area intro to “mean Earth radius”.
docs/api/flink/Geography-Functions.md	Updates Flink geography overview to “mean Earth radius”.

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[jiayuasu]

Reformatted to R = 6 371 008 m (inline code, space-separated) to match the ST_Area/ST_Length pages and the SedonaFlink geography docs. All six radius mentions across the geography docs now use the same style. Commit cf5385a.