clarify type requirements for search intersects parameter, update some documentation

[philvarner]

Related Issue(s): #146

Description:

1. For search intersects, tighten type definitions to require str, dict, or object with geo_interface attribute. Other objects now fail. If a user wants to use another object, they have to str() it first.
2. add better documentation about this.

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

Merging #174 (aca4f7c) into main (a12663a) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
+ Coverage   81.26%   81.39%   +0.13%     
==========================================
  Files           9        9              
  Lines         571      575       +4     
==========================================
+ Hits          464      468       +4     
  Misses        107      107              

Impacted Files	Coverage Δ
pystac_client/item_search.py	88.53% <100.00%> (+0.18%)	⬆️

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