address lesson feedback

concepts/conditionals/about.md

@@ -1,6 +1,9 @@
 # About
 
-Factor's conditionals all take quotations as their branches.
+Factor's conditionals all take **quotations** as their branches — code
+written in square brackets, `[ ... ]`, that is pushed as a value rather
+than run until the conditional selects it. (Quotations themselves are
+covered fully in a later exercise.)
 
 The basic three are in `kernel`:
 

concepts/conditionals/introduction.md

@@ -1,5 +1,11 @@
 # Introduction
 
+Factor's conditionals choose between **quotations** — snippets of code
+written in square brackets, `[ ... ]`. Writing `[ ... ]` pushes the
+code onto the stack as a value rather than running it; the conditional
+then decides which quotation to run. (Quotations are covered fully in a
+later exercise.)
+
 `if` (in [`kernel`][kernel]) is Factor's basic conditional. It
 takes a boolean and two quotations, running the first if the
 boolean is truthy and the second otherwise.

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

@@ -13,8 +13,9 @@ A literal array uses `{ }` with whitespace between elements:
 { "a" "b" "c" } .    ! => { "a" "b" "c" }
 ```
 
-Arrays are read-only — words like `suffix` and `remove-nth` return
-a *new* sequence rather than mutating the original.
+Arrays are fixed-length, so words like `suffix` and `remove-nth` —
+which change how many elements there are — return a *new* sequence
+rather than growing the original.
 
 ## Length and indexing
 

exercises/concept/cars-assemble/.docs/introduction.md

@@ -50,6 +50,22 @@ between? ( x lo hi -- ? )    ! lo <= x <= hi (inclusive)
 You'll often see it as a `cond` predicate — `dup 1 4 between?` —
 to pick a branch by range rather than by a single value.
 
+## Quotations
+
+The comparison words above produce booleans; to *act* on a boolean you
+hand the conditional one or more **quotations**. A quotation is a
+snippet of code wrapped in square brackets, `[ ... ]`. Writing it
+pushes the code onto the stack as a value instead of running it — a
+word like `if` then decides which quotation to run.
+
+```factor
+[ neg ]   ! a quotation that negates the top of the stack
+[ ]       ! the empty quotation — does nothing
+```
+
+A later exercise covers quotations in full; for now, read
+`[ ... ]` as "the code to run for this branch."
+
 ## `if`, `when`, `unless`
 
 `if` (in [`kernel`][kernel]) takes a boolean and two quotations. It