Add memory limit and frequency scaling info to Ardour manual

[sound8]

Linux

The memory warning (limits.conf) and frequency scaling warning (performance governor) that show up during Ardour install can be a little confusing. This is literally two of the first things users will experience when trying to install this software.
Not sure how common these two warnings are but it seems that some have mentioned experiencing these issues as well.
Not the best user experience to instantly experience issues/warnings that I imagine most users will not know anything about.

Suggesting to maybe consider adding some information about these two warnings into the Troubleshooting section of the manual.

I have read many posts here and elsewhere about these two issues for a while now and I still do not feel that I understand them fully, so I would imagine most new users will be even more confused about these warnings.

I plan on making other separate posts about these two issues/warnings to ask some questions about them but for this post just suggesting to consider adding some info about them to manual, if anyone agrees.

Thank You

[tseaver]

FWIW: the very helpful rtcqs project specifically aims to help users configure Linux systems for better audio latency / performance. When it detects a potential problem, it prints a warning, with a pointer to the appropriate reference documentation (usually from System configuration [Linux-Sound]) to explain more. For instance, see this message, printed when rtcqs detects that the user is not a member of a group granted rtprio permissions.

The obvious downside to this approach is the danger of linkrot.