fix get items recursive documentation

[mishaschwartz]

Related Issue(s):

• Closes Fix recursive search in Client.get_items #799

Description:

Clarifies get_items recursive behaviour as discussed in #799 (comment)

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to CHANGELOG.md

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.14%. Comparing base (21435b0) to head (b393214).
⚠️ Report is 183 commits behind head on main.

Files with missing lines	Patch %	Lines
pystac_client/client.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #800      +/-   ##
==========================================
+ Coverage   93.43%   94.14%   +0.71%     
==========================================
  Files          13       15       +2     
  Lines         990     1213     +223     
==========================================
+ Hits          925     1142     +217     
- Misses         65       71       +6     

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