Document pose + spherical coordinates combination on spawn

[chapulina]

🎉 New feature

Summary

Accompanies the implementation change in

• Optional orientation when spawning entity using spherical coordinates gz-sim#1263

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

Merging #212 (9fa5d0f) into ign-msgs8 (020ffb5) will not change coverage.
The diff coverage is n/a.

❗ Current head 9fa5d0f differs from pull request most recent head 7631d8a. Consider uploading reports for the commit 7631d8a to get more accurate results

@@            Coverage Diff             @@
##           ign-msgs8     #212   +/-   ##
==========================================
  Coverage      84.56%   84.56%           
==========================================
  Files              7        7           
  Lines            855      855           
==========================================
  Hits             723      723           
  Misses           132      132           

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-25-fortress-edifice-citadel/1343/1