Help: Format "if foreach while end..." docs; improve foreach" doc

• Presumes the syntax highlighting of !2468 (merged); applies it to the docs of commands "if", "foreach", "while", "break", "continue", "else", elseif", "endif", "endforeach", "endwhile".
• Makes consequential use of <..> placeholder in foreach doc.
• Relegates legacy argument of endforeach to a note in the text.
• Simplifies the explanations of the RANGE and IN signatures.
• Advises against negative arguments or reverse ranges in RANGE signatures; compare issue #18461
• Cross-reference from "foreach", "while" to "break", "continue"