Fix doc comment for `geos.ConcaveHull`

[peterstace]

Description

Godoc was rendering the doc comment in a bit of a weird way. This commit fixes that (and also adds a bit more detail).

Check List

Have you:

• Added unit tests? N/A

• Add cmprefimpl tests? (if appropriate?) N/A

• Updated release notes? (if appropriate?) N/A

Related Issue

• N/A

[peterstace]

Thanks @albertteoh ! 😁