[WIP] ESQL docs: clarify duration aliases (minute vs month), case-insensitive, optional whitespace

[Zachary-hub]

This is a docs-only change clarifying duration aliases in ESQL.

• month: m/mm; minute: min/mi/n; millisecond: ms
• matching is case-insensitive
• whitespace between number and unit is optional (1m == 1 m)
• examples included

Rationale: adding m as an alias for minute would be ambiguous with m for month
(case-insensitive matching + optional whitespace). Therefore we clarify the docs
and keep the current aliasing scheme.

Relates: #135552

Credit to @Wentao14614 for the initial observation.

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💚 CLA has been signed

[github-actions]

🔍 Preview links for changed docs

• docs/reference/query-languages/esql/esql-time-spans.md

[github-actions]

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