Some commands still use word player instead of entity

Affected commands:

• /execute @p ~ ~ ~ /kill You must...which player... (see also MC-78779)
• Command block GUI "@r to target random player" (might be wrong as you can add type=!Player)
• /help spreadplayers
• /help effect
• /help testfor
• /help tp

Note that it is also inconsistent, for example:
/help kill: /kill [player|entity] (It is a little bit irritating as commands like /replaceitem have <block|entity> as argument)
/help execute: /execute <entity>