Media: Clarify caption field labels for images and videos

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/43178

Fixes the ambiguity of the "Caption" label in media attachment fields by rendering a context-aware label based on media type.

• Images display Image Caption, aligning with the established photography term.
• Videos display Video Description, avoiding the broadcast-industry meaning of "caption" (i.e. subtitle/SRT tracks).
• All other media types (audio, documents, etc.) retain the existing Caption label.

Before	After

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