doc: improve triaging text in issues.md
Remove a double negative and many superfluous sentences. The sentiment in the removed material is good, but we already tell people that discussion should be focused, helpful, and professional. Having two extra paragraphs greatly reduces the likelihood that people will read the material.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines