docs: fix init epilog about what none mode can (not) detect (1.4-maint)

[ThomasWaldmann]

The "About modes without encryption" paragraph of the borg init epilog (also in the generated borg-init man page) says that using any mode other than none "allows Borg to detect accidental corruption or malicious tampering with the repo", which implies none mode detects neither.

That is only half true. none mode (PlaintextKey) still:

• verifies the CRC32 of every repository object on read and in the repository check,
• verifies the SHA-256 chunk IDs in decrypt() (assert_id), i.e. on extract, mount, borg check --verify-data, etc.,
• verifies the manifest/archive TAMs, but with a key derived from public data only.

So accidental corruption is detected. What none mode cannot detect is malicious tampering, because none of these checks involve a secret key: anyone with write access can modify data and recompute everything up the chain (or simply run borg against the repo). The borg check epilog already states this correctly ("Tamper-resistance is only guaranteed ... against attackers without access to the keys").

Verified on a 1.4-maint build with -e none repos:

Scenario	borg check	check --verify-data	extract
flip one data byte in a segment	segment entry checksum mismatch	same	IntegrityError
flip one byte and recompute the CRC32	passes (plain check never reads content chunks, in any mode)	id verification failed	id verification failed
borg recreate --exclude without any key/passphrase	clean	clean	n/a

This PR only rewords that paragraph; the generated docs/usage/init.rst.inc and man page are left for the usual release-time regeneration.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.13%. Comparing base (d2fb419) to head (8e13924).
⚠️ Report is 1 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint   #10343      +/-   ##
=============================================
- Coverage      82.13%   82.13%   -0.01%     
=============================================
  Files             38       38              
  Lines          11478    11478              
  Branches        1808     1808              
=============================================
- Hits            9428     9427       -1     
- Misses          1465     1466       +1     
  Partials         585      585              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.