[ThinLTO] In documentation, mention possible values for concurrency flags

[aganea]

Summary

As suggested here: https://reviews.llvm.org/D75153#2323939

[hans]

Thanks for documenting these!

I wonder if including the "CPU sockets" part is more confusing than helpful (especially if the reader doesn't know about CPU sockets). The sockets don't really matter here anyway.

Sometimes the term "physical core" is used in relation to hyper-threading and I find that pretty understandable. How about something line:

• 0: Use one thread per physical core (default)
• 1: Use a single thread only (disable multi-threading)
• all: Use one thread per logical core (uses all hyper threads)

[aganea]

As suggested by @hans.

clang/docs/ThinLTO.rst

- gold:
   ``-Wl,-plugin-opt,jobs=N``
 - ld64:
   ``-Wl,-mllvm,-threads=N``
 - lld:
   ``-Wl,--thinlto-jobs=N``
 - lld-link:
   ``/opt:lldltojobs=N``
 
+Other possible values for ``N`` are:
+- 0: Use one thread per physical core (default)
+- 1: Use a single thread only (disable multi-threading)
+- all: Use one thread per logical core (uses all hyper-threads)
+
 Incremental
 -----------
 .. _incremental:
 
 ThinLTO supports fast incremental builds through the use of a cache,
 which currently must be enabled through a linker option.
 
 - gold (as of LLVM 4.0):