Document platform specific limitations with shared memory

[Dalzhim]

It can be very hard to figure out why shared memory cannot be created on macos when getting errno=1 ("Operation not permitted") as the only information. It turns out that sandboxed applications have extra hoops to go through to use shared memory. Here are the relevant documents :

• https://developer.apple.com/library/archive/documentation/Security/Conceptual/AppSandboxDesignGuide/AppSandboxInDepth/AppSandboxInDepth.html#//apple_ref/doc/uid/TP40011183-CH3-SW24
• https://developer.apple.com/library/archive/documentation/Security/Conceptual/AppSandboxDesignGuide/AppSandboxInDepth/AppSandboxInDepth.html#//apple_ref/doc/uid/TP40011183-CH3-SW21
• https://developer.apple.com/library/archive/documentation/Miscellaneous/Reference/EntitlementKeyReference/Chapters/EnablingAppSandbox.html#//apple_ref/doc/uid/TP40011195-CH4-SW19

I believe the interprocess library's documentation could point people to Apple's documentation in order to make it easier to use!