Move "hardcoded" notes from metric semconv md files to the notes field in YAML

[joaopgrassi]

The build tools does not render today the note field on a metric defined in yaml. This has caused some of the semconv markdown files to include notes "hardcoded" in the markdown files.

When open-telemetry/build-tools#177 is merged, the tool will then properly render the notes and link it to the description column in the metrics table, same way done for attribute notes.

We then need to move the existing notes from the markdown file to their yaml definitions and re-render the markdown so the notes are then not hardcoded anymore.

I looked and I only found ones that need changing in the runtime-environment-metrics.md, but things might change with incoming PRs so need to double check when work on this starts