docs: correct tcp_connection_remote label documentation

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Description

Fixes #2559.

The docs describe tcp_connection_remote as a per-remote-endpoint metric with address (IP:port) labels, but that hasn't been true since #953, which deliberately dropped the port label and folded every remote socket into a single address="AllIPs" series to keep the metric's cardinality bounded.

So this is a docs bug rather than a code bug - the aggregation is the intended behavior. This PR takes option 2 from the issue and makes the documentation match what the exporter actually emits:

• docs/03-Metrics/02-hubble_metrics.md and docs/03-Metrics/modes/basic.md: label column now says address is always AllIPs, plus a short note in the basic-mode label values section explaining why individual remotes aren't exported.
• docs/06-Troubleshooting/basic-metrics.md: the sample scrape output still showed per-IP series with a port label that no longer exists. Replaced with the single aggregated series.
• pkg/metrics/types.go: the gauge's HELP string said "by remote address", which is the same misleading claim shipped to every Prometheus scrape. Reworded.
• pkg/plugin/linuxutil/netstat_stats_linux.go: added a note on the addrDefaultTCPRemote constant so the aggregation doesn't read like an unfinished implementation next time.

Worth calling out for the issue author: this closes the docs mismatch but does not give you the per-remote breakdown you were after. If there's appetite for that as an opt-in (default stays aggregated, a config flag turns on per-endpoint series with stale-series cleanup between polls), I'm happy to follow up with a separate PR - but that seemed like a decision for the maintainers rather than something to sneak into a docs fix.

Related Issue

#2559

Checklist

• I have read the contributing documentation.
• I signed and signed-off the commits (git commit -S -s ...).
• I have correctly attributed the author(s) of the code.
• I have tested the changes locally.
• I have followed the project's style guidelines.
• I have updated the documentation, if necessary.
• I have added tests, if applicable.

Screenshots (if applicable) or Testing Completed

No behavior change, so nothing new to test - go build ./pkg/metrics/... ./pkg/plugin/linuxutil/... passes and the only runtime-visible difference is the HELP text on the gauge.

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