Added note to highlight that some environment variables can't be set …

…in `startup.jl` (JuliaLang#30015)

doc/src/manual/environment-variables.md

@@ -14,6 +14,13 @@ The environment variables that Julia uses generally start with `JULIA`. If
 output will list defined environment variables relevant for Julia, including
 those for which `JULIA` appears in the name.
 
+!!! note
+
+ Some variables, such as `JULIA_NUM_THREADS` and `JULIA_PROJECT` need to be set before Julia
+ starts, therefore adding these to `~/.julia/config/startup.jl` is too late in the startup process.
+ These must either be set manually before launching Julia through bash with
+ `export JULIA_NUM_THREADS=4` etc. or added to `-/.bashrc` and/or `~/.bash_profile` to achieve persistence.
+
 ## File locations
 
 ### `JULIA_BINDIR`
@@ -74,6 +81,10 @@ Julia tries to find a project directory that contains `Project.toml` or
 `JuliaProject.toml` file from the current directory and its parents. See also
 the chapter on [Code Loading](@ref).
 
+!!! note
+
+ `JULIA_PROJECT` must be defined before starting julia; defining it in `startup.jl` is too late in the startup process.
+
 ### `JULIA_LOAD_PATH`
 
 A separated list of absolute paths that are to be appended to the variable
@@ -152,6 +163,10 @@ physical CPU cores, then the number of threads is set to the number of cores. If
 cores cannot be determined through system calls, then the number of threads is
 set to `1`.
 
+!!! note
+
+ `JULIA_NUM_THREADS` must be defined before starting julia; defining it in `startup.jl` is too late in the startup process.
+
 ### `JULIA_THREAD_SLEEP_THRESHOLD`
 
 If set to a string that starts with the case-insensitive substring `"infinite"`,