Simplify recursion introduction

concepts/recursion/about.md

@@ -17,8 +17,6 @@ A few things to know:
 
 - Plain `:` definitions can recurse with no special syntax. The
   compiler infers stack effects across the recursive call.
-- `inline` words that recurse need the additional `recursive`
-  modifier so the compiler knows the recursion is intentional.
 - `loop ( quot -- )` lets you iterate a quotation that returns a
   boolean — keep going while it's truthy.
 - Tail calls compile to jumps, so deeply recursive words don't

exercises/concept/backyard-birdcount/.docs/introduction.md

@@ -6,9 +6,7 @@ spell a computation is **recursion** — a word that calls itself.
 
 ## Defining a recursive word
 
-A `:` definition can call itself directly. The compiler can usually
-infer the stack effect; if it can't, you can add the `recursive`
-modifier to ask it to assume self-consistency:
+A `:` definition can call itself directly:
 
 ```factor
 : count-down ( n -- )

exercises/concept/boutique-bookkeeping/.docs/introduction.md

@@ -212,6 +212,11 @@ USING: math sequences ;
 Words built only from literal quotations don't need `inline` —
 just the ones that pass an *incoming* quotation through.
 
+If such an `inline` word ever calls *itself* — a recursive
+combinator — declare it `; inline recursive` instead. Without the
+extra word the compiler stops with *"The inline recursive word ...
+must be declared recursive"*, which tells you exactly what to add.
+
 ## Pulling the second element
 
 For a 2-element array, `second` returns its second element. `first`