Revise jldoctest best practices

Author: Keno

doc/make.jl

@@ -217,6 +217,7 @@ DevDocs = [
         "devdocs/functions.md",
         "devdocs/cartesian.md",
         "devdocs/meta.md",
+        "devdocs/jldoctests.md",
         "devdocs/subarrays.md",
         "devdocs/isbitsunionarrays.md",
         "devdocs/sysimg.md",

doc/src/devdocs/jldoctests.md

@@ -0,0 +1,76 @@
+# Best practices for jldoctests
+This page describes how to write and maintain `jldoctest` blocks in the documentation. Following these guidelines helps keep doctests reliable and easy to read.
+
+
+## Filters
+
+Use `filter =` whenever output contains text that might vary across runs.
+The documentation relies on several recurring patterns:
+
+- `r"int.jl:\\d+"` — remove line numbers from introspection macros.
+- `r"Stacktrace:(\\n \\[0-9]+\\].*)*"` — hide stack traces when illustrating
+  errors.
+- `r"Closest candidates.*\\n  .*"` — skip the method suggestions printed by
+  `MethodError`.
+- `r"@ .*"` — strip file locations from the output of `methods` or
+  `@which`.
+- `r"\\@world\\(MyStruct, \\d+:\\d+\\)"` — filter world age numbers.
+- `r"with \\d+ methods"` — ignore method counts when redefining functions.
+- `r"[0-9\\.]+ seconds \\(.*?\\)"` — remove timing output with memory
+  information.
+- `r"[0-9\\.]+ seconds"` — remove simple timing results.
+- `r"[0-9\\.]+"` — filter digits from names such as anonymous functions.
+- `r"([A-B] [0-5])"` and `r"[A-B] [X-Z] [0-5]"` — account for non-deterministic
+  process output.
+- `r"(world\\nhello|hello\\nworld)"` — allow either ordering of interleaved
+  output.
+
+If none of these match your situation, craft a regular expression that
+removes the varying text. Using filters keeps doctests stable across
+platforms and Julia versions.
+
+## Setup code
+
+Small setup expressions may be placed inline using the `setup =` option:
+
+```jldoctest; setup = :(using InteractiveUtils)
+...
+```
+
+For longer setup code or if multiple blocks require the same environment, use the
+`DocTestSetup` meta block:
+
+```@meta
+DocTestSetup = :(import Random; Random.seed!(1234))
+```
+
+and disable it afterwards with
+
+```@meta
+DocTestSetup = nothing
+```
+
+## Maintaining state between snippets
+
+Related doctest blocks can share state by giving them the same label after the
+`jldoctest` keyword. The manual uses this pattern to demonstrate mutation:
+
+```jldoctest mutation_vs_rebind
+julia> a = [1,2,3]
+...
+```
+
+and later
+
+```jldoctest mutation_vs_rebind
+julia> a[1] = 42
+...
+```
+
+Blocks with the same name execute sequentially during doctesting, so variables
+created in the first block remain available in the following ones.
+
+When a snippet needs to preserve its result for later examples, give it a label
+and reuse that label. This avoids repeating setup code and mirrors a REPL
+session more closely.
+