Multithreading.md does not introduce include file needed for function calls

[Barry Smith]

Multithreading.md should, above the first mention of using C function calls to set the number of threads, indicate what include file needs to be included.

[Field G. Van Zee]

How does this look, @BarrySmith?

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Globally at runtime

Note: If you want to gain access to BLIS API function prototypes, be sure to #include "blis.h" from the relevant source files in your application.

If you still wish to set the parallelization scheme globally, but you want to do so at runtime, BLIS provides a thread-safe API for specifying multithreading. Think of these functions as a way to modify the same internal data structure into which the environment variables are read. (Recall that the environment variables are only read once, when BLIS is initialized).