A few improvements to control flow section (#907)

* control-flow: blocks: clarify prose around block values/types

specify what determines the type of a block

move the last expression note to the discussion of blocks rather than functions to clarify that it applies to both

* control-flow: if-let: correct sense of 'non-returning'

src/control-flow/blocks.md

@@ -1,7 +1,8 @@
 # Blocks
 
-A block in Rust has a value and a type: the value is the last expression of the
-block:
+A block in Rust contains a sequence of expressions.
+Each block has a value and a type,
+which are those of the last expression of the block:
 
 ```rust,editable
 fn main() {
@@ -22,6 +23,8 @@ fn main() {
 }
 ```
 
+If the last expression ends with `;`, then the resulting value and type is `()`.
+
 The same rule is used for functions: the value of the function body is the
 return value:
 
@@ -35,8 +38,6 @@ fn main() {
 }
 ```
 
-However if the last expression ends with `;`, then the resulting value and type is `()`.
-
 <details>
 
 Key Points:

src/control-flow/if-let-expressions.md

@@ -23,7 +23,7 @@ Rust.
 * Unlike `match`, `if let` does not have to cover all branches. This can make it more concise than `match`.
 * A common usage is handling `Some` values when working with `Option`.
 * Unlike `match`, `if let` does not support guard clauses for pattern matching.
-* Since 1.65, a similar [let-else](https://doc.rust-lang.org/rust-by-example/flow_control/let_else.html) construct allows to do a destructuring assignment, or if it fails, have a non-returning block branch (panic/return/break/continue):
+* Since 1.65, a similar [let-else](https://doc.rust-lang.org/rust-by-example/flow_control/let_else.html) construct allows to do a destructuring assignment, or if it fails, execute a block which is required to abort normal control flow (with `panic`/`return`/`break`/`continue`):
 
    ```rust,editable
    fn main() {