docs: document that borg transfer is incremental

[ThomasWaldmann]

Fixes #9878.

borg transfer already is incremental, but nothing in the docs said so, so users
have to guess whether re-running it (e.g. from a cron job) is safe and cheap.

This adds an "Incremental transfer" section to the borg transfer command help
(and thus to docs/usage/transfer.rst.inc) covering:

• archive level: archives already present in the destination repo are skipped
  (checked by name+timestamp and by name+id), printing
  archive is already present in destination repo, skipping.
• chunk level: for archives that do get copied, only chunks not yet present in
  the destination are read from the source and stored.
• the cron / multiple-invocations-with--a-filters pattern from the issue.
• interrupting a transfer is safe: an archive only shows up in the destination
  after it was copied completely, and the chunks stored so far are reused on the
  next run - unless borg compact runs on the destination in between.
• transfer never deletes: prune/compact the destination repo separately.

Also adds a pointer from the related FAQ entry ("Can I copy or synchronize my
repo to another location?") to borg transfer.

docs/usage/transfer.rst.inc was regenerated with scripts/make.py build_usage
(only that one file changed); man pages were not regenerated, that is done at
release time.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.25%. Comparing base (d7b1cd5) to head (1dc0525).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Coverage   87.25%   87.25%           
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  Files         102      102           
  Lines       18440    18440           
  Branches     2834     2834           
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  Hits        16090    16090           
  Misses       1643     1643           
  Partials      707      707           

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