add debug mode docs

Author: oxinabox

docs/make.jl

@@ -11,7 +11,7 @@ DocMeta.setdocmeta!(
         Random.seed!(0)  # frule doctest shows output
 
         using ChainRulesCore
-        # These rules are all actually defined in ChainRules.jl, but we redefine them here to 
+        # These rules are all actually defined in ChainRules.jl, but we redefine them here to
         # avoid the dependency.
         @scalar_rule(sin(x), cos(x))  # frule and rrule doctest
         @scalar_rule(sincos(x), @setup((sinx, cosx) = Ω), cosx, -sinx)  # frule doctest
@@ -28,6 +28,7 @@ makedocs(
         "Introduction" => "index.md",
         "FAQ" => "FAQ.md",
         "Writing Good Rules" => "writing_good_rules.md",
+        "Debug Mode" => "debug_mode.md",
         "API" => "api.md",
     ],
     strict=true,

docs/src/api.md

@@ -30,4 +30,5 @@ Private = false
 ## Internal
 ```@docs
 ChainRulesCore.AbstractDifferential
+ChainRulesCore.debug_mode
 ```

docs/src/debug_mode.md

@@ -1,13 +1,13 @@
 # Debug Mode
 
-ChainRules supports a [`debug_mode`](@ref) which you can use while writing new rules.
+ChainRules supports a *debug mode* which you can use while writing new rules.
 It provides better error messages.
 If you are developing some new rules, and you get a weird error message,
 it is worth enabling debug mode.
 
 There is some overhead to having it enabled, so it is disabled by default.
 
-To enable redefine the `debug_mode()` function to return `true`.
+To enable redefine the []`ChainRulesCore.debug_mode`](@ref) function to return `true`.
 ```julia
 ChainRulesCore.debug_mode() = true
 ```