Add Rhai book.

doc/src/language/statements.md

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+Statements
+==========
+
+{{#include ../links.md}}
+
+Statements are terminated by semicolons '`;`' and they are mandatory,
+except for the _last_ statement in a _block_ (enclosed by '`{`' .. '`}`' pairs) where it can be omitted.
+
+A statement can be used anywhere where an expression is expected. These are called, for lack of a more
+creative name, "statement expressions."  The _last_ statement of a statement block is _always_ the block's
+return value when used as a statement.
+If the last statement has no return value (e.g. variable definitions, assignments) then it is assumed to be [`()`].
+
+```rust
+let a = 42;             // normal assignment statement
+let a = foo(42);        // normal function call statement
+foo < 42;               // normal expression as statement
+
+let a = { 40 + 2 };     // 'a' is set to the value of the statement block, which is the value of the last statement
+//              ^ the last statement does not require a terminating semicolon (although it also works with it)
+//                ^ semicolon required here to terminate the assignment statement; it is a syntax error without it
+
+4 * 10 + 2              // a statement which is just one expression; no ending semicolon is OK
+                        // because it is the last statement of the whole block
+```