Explain perf keywords (#7307)
Explain perf keywords (#7307)

These have specific meanings but they weren't written down in an easy form
so it was easy to misuse them especially via copy/paste. Now they'll show
up in hover.
sean-gilliam/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild