docs(escrow): update the module doc comment for crowdfunding

[chmm195]

Closes #157.

Comment-only change to contracts/escrow/src/lib.rs; no code touched.

One note on the wording. #157 suggests matching milestones's comment, but the two contracts refund differently, so I did not copy it across:

• milestones refunds the unallocated remainder "in proportion to what they put in" — proportional, because the budget is allocated to issues in slices over time, so there may be less left than was contributed.

• escrow has no partial allocation. refund iterates 0..contributor_count and transfers each Contribution its own stored amount:

  token_client.transfer(
      &contract_address,
      &contribution.sponsor,
      &contribution.amount,
  );

  So every contributor gets back exactly what they put in. Saying "in proportion" here would be true but weaker than the guarantee the code actually makes, so the comment says "the exact amount they put in".

I also reworded the opening line: "sponsor-funded" read as one sponsor per issue, so it now names the fund / contribute split explicitly.

Left release's and fund's own doc comments alone — fund's was already updated in 4279873.

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