Documentation: Explain --config.expand-env=true double slash with slash substitution

[V-Mann-Nick]

What this PR does / why we need it:

When I was configuring Promtail to use a regex stage I escaped slashes in regular expressions when using double quotes in yaml files. But no where did it explain that when using the option --config.expand-env=true the program envsubst would replace double slashes with slashes. I found the issue #3635 to find out this is behavior as expected. While I accept this, I thought it would be useful to warn the next person about just this case.

Adds two paragraphs to the Promtail configuration docs:

• in docs/sources/clients/promtail/configuration.md: use double slashes for single slashes when using expand-env=true
• in docs/sources/clients/promtail/stages/regex.md: explained again as it's more likely to use back slashes in regex

Which issue(s) this PR fixes:

Although already closed it's related to the following: Fixes #3635

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[CLAassistant]

All committers have signed the CLA.

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[kavirajk]

thanks @V-Mann-Nick 👍 LGTM. Added one small suggestion with linking reference.

[V-Mann-Nick]

Thanks for the suggestions @kavirajk. Didn't know that. I committed them.

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[kavirajk]

Thanks @V-Mann-Nick LGTM