Amend built-in documentation

Various minor fixes across the built-in documentation

yash-builtin/src/shift.rs

@@ -26,8 +26,8 @@
 //!
 //! # Description
 //!
-//! The built-in removes the first `n` positional parameters from the list of
-//! positional parameters. If `n` is omitted, it is assumed to be `1`.
+//! The built-in removes the first *n* positional parameters from the list of
+//! positional parameters. If *n* is omitted, it is assumed to be `1`.
 //!
 //! # Options
 //!
@@ -54,12 +54,6 @@
 //! runtime error. This implementation treats it as a syntax error.
 //!
 //! (TODO: the array option and negative operands)
-//!
-//! # Implementation notes
-//!
-//! This built-in expects the [positional
-//! parameters](yash_env::variable::VariableSet::positional_params_mut) to be an
-//! array. If it is not an array, the built-in panics.
 
 use crate::common::arrange_message_and_divert;
 use crate::common::syntax_error;

yash-builtin/src/unset.rs

@@ -16,7 +16,7 @@
 
 //! Unset built-in
 //!
-//! The **`unset`** built-in unsets the values of shell variables.
+//! The **`unset`** built-in unsets shell variables or functions.
 //!
 //! # Synopsis
 //!
@@ -48,7 +48,7 @@
 //!
 //! # Errors
 //!
-//! Unsetting a read-only variable is an error.
+//! Unsetting a read-only variable or function is an error.
 //!
 //! It is not an error to unset a variable or function that is not set.
 //! The built-in ignores such operands.

yash-builtin/src/wait.rs

@@ -46,7 +46,8 @@
 //!
 //! ## Job ID
 //!
-//! TODO Elaborate on syntax of job ID
+//! A job ID must start with `%` and has the format described in the
+//! [`yash_env::job::id`] module documentation.
 //!
 //! ## Process ID
 //!