[Sync EN] Fix grammar in language/predefined (#5755)

[lacatoire]

Syncs the Spanish translation with php/doc-en#5755 (commit 029f19d).

• The eight touched intro paragraphs become simpara, as in EN.
• serializable.xml: paragraph rewritten to match the English wording (method names tagged with methodname, and the constructor can be called from within the unserialize method).
• stdclass.xml: "con el operador new" instead of "con new operador".
• traversable.xml: semicolon instead of comma, as in EN.
• weakmap.xml: example title fixed to WeakMap.
• weakreference.xml: "estructuras de tipo caché" (cache-like structures).
• EN-Revision updated in the eight files.

Fixes: #968

Also included, because the structure and EN-Revision checks require it on the touched files: the xi:include elements are aligned with doc-en (self-closing, no xi:fallback), and traversable.xml is synced with php/doc-en#5773 (commit 14013fe): changelog row 7.4.0 corrected to 8.0.0, notes turned into simpara and reworded (prior to PHP 8.0.0 the interface could not be implemented directly, not even by abstract classes).