Update README.md to clarify data-mounts

[hotzen]

clarify the intent of the data-mounts that are provided as examples

If this is a fix for a typo (in code, documentation, or the README) please file an issue and let us sort it out. We do not need a PR

Hope this PR is so low-effort in reviewing & merging that it can be integrated despite the closed issue: #57

There are quite a few questions around this when googling/perplexity'ing the internetz, so I guess a bit of clarification in the documentation is beneficial.

Thanks for the great containers

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[Roxedus]

Please read CONTRIBUTING.md

[LinuxServer-CI]

I am a bot, here are the test results for this PR:
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https://ci-tests.linuxserver.io/lspipepr/syncthing/v1.28.1-pkg-6c5a413b-dev-13433e817e40533cb86286aafeade338a506e9f1-pr-83/shellcheck-result.xml

Tag	Passed
amd64-v1.28.1-pkg-6c5a413b-dev-13433e817e40533cb86286aafeade338a506e9f1-pr-83	✅
arm64v8-v1.28.1-pkg-6c5a413b-dev-13433e817e40533cb86286aafeade338a506e9f1-pr-83	✅

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I am a bot, here are the test results for this PR:
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https://ci-tests.linuxserver.io/lspipepr/syncthing/v1.29.2-pkg-4ebfafd4-dev-96783ff9312cca39f1cf452869178f4b00ccbb18-pr-83/shellcheck-result.xml

Tag	Passed
amd64-v1.29.2-pkg-4ebfafd4-dev-96783ff9312cca39f1cf452869178f4b00ccbb18-pr-83	✅
arm64v8-v1.29.2-pkg-4ebfafd4-dev-96783ff9312cca39f1cf452869178f4b00ccbb18-pr-83	✅

[Roxedus]

Closing due to lack of response.