Improve documentation for JULIA_NUM_THREADS=auto (#42501)

Co-authored-by: Jameson Nash <[email protected]>
Co-authored-by: Fredrik Ekre <[email protected]>

Author: 3 people

doc/src/base/multi-threading.md

@@ -8,6 +8,7 @@ Base.Threads.threadid
 Base.Threads.nthreads
 ```
 
+See also [Multi-Threading](@ref man-multithreading).
 ## Synchronization
 
 ```@docs

doc/src/manual/multi-threading.md

@@ -18,10 +18,16 @@ The number of execution threads is controlled either by using the
 [`JULIA_NUM_THREADS`](@ref JULIA_NUM_THREADS) environment variable. When both are
 specified, then `-t`/`--threads` takes precedence.
 
+The number of threads can either be specified as an integer (`--threads=4`) or as `auto`
+(`--threads=auto`), where `auto` sets the number of threads to the number of local CPU
+threads.
+
 !!! compat "Julia 1.5"
     The `-t`/`--threads` command line argument requires at least Julia 1.5.
     In older versions you must use the environment variable instead.
 
+!!! compat "Julia 1.7"
+    Using `auto` together with the environment variable `JULIA_NUM_THREADS` requires at least Julia 1.7.
 Lets start Julia with 4 threads:
 
 ```bash