Clarify min and max temperature of thermal camera (backport #523)

[mergify]

This is an attempt to improve the documentation of the <min_temp>, <max_temp> and <resolution> tags in the thermal camera documentation.

After reading the description of <min_temp> and <max_temp> in the thermal camera tutorial I expected an output camera image value of 0 to mean <min_temp> Kelvin and a value of 2^bitDepth - 1 to mean <max_temp> Kelvin, so I was surprised to find out that that was not the case. Granted, the presence of the <resolution> tag did not really fit this mental model. It is also possible that this confusion was amplified by bugs that are now fixed (gazebosim/gz-rendering#1133 and gazebosim/gz-rendering#1138).

Nevertheless I tried to rephrase the descriptions a bit in an attempt to minimize the potential for confusion in the future. Hopefully it makes some sense to people other than me.

Checklist

• Signed all commits for DCO
• Updated documentation (as needed)

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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This is an automatic backport of pull request #523 done by Mergify.