docs: an oversized upscale is capped, not refused - #68

Follows rendobar/rendobar#465 and #467, which changed the behaviour after these docs shipped.

Was: an over-large request failed with VALIDATION_ERROR before the model loaded.

Now: it renders the largest size that fits, keeps the aspect ratio, and logs what it did.

Fixes three stale claims:

• "the output lands on exactly the size you asked for" was unconditional. It now says "as long as it fits" and links to Size limits.
• The whole Size limits section described the refusal and its error message. Replaced with the capping behaviour, the real log line, and a note to read output.file.meta rather than assuming the dimensions match the request.
• Adds that the limit is on output pixels, not on the factor, which is why a 1365x2048 source caps below 2x while a small source reaches 4x. This is the part that confused a real user.

Also documents that seedvr2-7b caps roughly 40% sooner than seedvr2-3b, per the per-checkpoint ceiling in #465.

Validator: 22 files, all valid.