Mention Base.Lockable in "multi-threading.md"

[KronosTheLate]

When I originally learned about multithreading and locks (and ended up rewriting [https://docs.julialang.org/en/v1/manual/multi-threading/#man-using-locks](Using locks to avoid data-races)), I was puzzled to find that the association of a lock and a value was the mental task of the programmer, and not the programmatic task of the program. I made that explicit in that section of the manual, writing the following:

Note that the link between a lock and a variable is made by the programmer, and not the program.

I was therefore very happy to see Base.Lockable introduced in Julia 1.11. What was missing, was any mention of it in the relevant section of the manual.

This PR adds a subsection under locks (So a forth level of headings, not sure if that is fine), showcasing and reccomending the use of Base.Lockable. I have made heavy use of comments in the example, which breaks with the general style, but adds valuable interpretation along the way. I am very open restructuring that part in particular.

[nsajko]

how far back can this be backported?

[KronosTheLate]

I am almost certain that Lockable was introduced in 1.11.

[KronosTheLate]

Can confirm that Base.Lockable is not defined in 1.10, but is in 1.11.

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.11.4 (2025-03-10)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> Base.Lockable
Base.Lockable

julia>

󰍲 dennis.bal  ~   v3.12.10   08:09 
 julia +1.10
ERROR: `1.10` is not installed. Please run `juliaup add 1.10` to install channel or version.

󰍲 dennis.bal  ~   v3.12.10   08:09 
 juliaup add 1.10
Checking for new Julia versions
Installing Julia 1.10.9+0.x64.w64.mingw32

󰍲 dennis.bal  ~   v3.12.10   08:10 
 julia +1.10
Precompiling OhMyREPL finished.
  8 dependencies successfully precompiled in 17 seconds
[ Info: OhMyREPL loaded
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.10.9 (2025-03-10)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> Base.Lockable
ERROR: UndefVarError: `Lockable` not defined
Stacktrace:
 [1] getproperty(x::Module, f::Symbol)
   @ Base .\Base.jl:31
 [2] top-level scope
   @ REPL[1]:1

[KronosTheLate]

Is this PR waiting for anything in particular?

[KristofferC]

Some comments about this code block:

• I think it is overcommenting a bit. There is not really a need to comment on creating an empty array.
• The comments become quite long on a single line, which will require horizontal scrolling to read on the website.

I'd take some inspiration of how the section below is written, ideally the text before the code should be enough to understand the code.