RFR (S): 6516521: Doc: should document about the primordial thread attaching to the VM

[David Holmes]

Although the JNI spec itself is not an open document I see no reason not 
to get feedback on this simple update.

Bug: https://bugs.openjdk.java.net/browse/JDK-6516521

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Chapter 5: The Invocation API

Creating the VM

The JNI_CreateJavaVM() function loads and initializes a Java VM and 
returns a pointer to the JNI interface pointer. The thread that called 
JNI_CreateJavaVM() is considered to be the main thread.
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Proposed additional text:

*Note:* Depending on the operating system, the primordial process thread 
may be subject to special handling that impacts its ability to function 
properly as a normal Java thread (such as having a limited stack size 
and being able to throw `StackOverflowError`). It is strongly 
recommended that the primordial thread is not used to load the Java VM, 
but that a new thread is created just for that purpose.

Thanks,
David