settype(): document that it always returns true and can throw TypeError - #5916

The return values section used return.success, which says the function
returns false on failure. It cannot return false. I tried an invalid type
name, an array, an object and a resource, and a reference to a typed
property, and it either returned true or threw.

Switched to return.true.always, the entity already used by openlog(),
closelog() and syslog() for the same situation.

The errors section only documented the ValueError for an invalid type
name. A TypeError is also thrown when var is a reference to a typed
property and the converted value cannot be assigned back to the declared
type. Tested on PHP 8.4.26:

  declare(strict_types=1);
  class Foo { public int $value = 123; }
  $foo = new Foo;
  $ref = &$foo->value;
  settype($ref, "string");
  // TypeError: Cannot assign string to reference held by property
  //            Foo::$value of type int

I wrote it as "cannot be assigned under the active typing mode" instead
of listing the cases, because widening still works in strict mode. With
strict_types a float property set to "integer" returns true and ends up
as float(1), while an int property set to "float" throws.

Added a note for the coercive case, where the converted value is assigned
back and gets coerced to the declared type, so the property keeps its
type and the call still returns true. That is the behaviour reported in
the issue that looked like a bug.

Fixes php#2677

Apply @lacatoire's wording suggestion for the note and switch the three
inline-only para blocks to simpara, which the DocBook style check flagged.