docs: fix markdown lint issues (#995)

* fix list indents in notifications
* fix missing lang tag
* fix list indent and extra newlines
* further indentation experimentation
* fix remark lint-recommend warnings

docs/arguments.md

@@ -258,7 +258,7 @@ Environment Variable: WATCHTOWER_HTTP_API_TOKEN
 ## HTTP API periodic polls
 Keep running periodic updates if the HTTP API mode is enabled, otherwise the HTTP API would prevent periodic polls.  
 
-```
+```text
             Argument: --http-api-periodic-polls
 Environment Variable: WATCHTOWER_HTTP_API_PERIODIC_POLLS
                 Type: Boolean

docs/container-selection.md

@@ -54,5 +54,3 @@ docker run -d --label=com.centurylinklabs.watchtower.monitor-only=true someimage
 ```
 
 When the label is specified on a container, watchtower treats that container exactly as if [`WATCHTOWER_MONITOR_ONLY`](https://containrrr.dev/watchtower/arguments/#without_updating_containers) was set, but the effect is limited to the individual container. 
-
-

docs/notifications.md

@@ -198,7 +198,7 @@ outputs timestamp and log level.
     If you want to adjust the date/time format it must show how the
     [reference time](https://golang.org/pkg/time/#pkg-constants) (_Mon Jan 2 15:04:05 MST 2006_) would be displayed in your
     custom format.  
-    i.e. The day of the year has to be 1, the month has to be 2 (february), the hour 3 (or 15 for 24h time) etc.
+    i.e., The day of the year has to be 1, the month has to be 2 (february), the hour 3 (or 15 for 24h time) etc.
 
 Example:
 

docs/private-registries.md

@@ -87,7 +87,6 @@ services:
       - /var/run/docker.sock:/var/run/docker.sock
 ```
 
-
 ## Credential helpers
 Some private Docker registries (the most prominent probably being AWS ECR) use non-standard ways of authentication.
 To be able to use this together with watchtower, we need to use a credential helper.
@@ -98,7 +97,6 @@ helper in a separate container and mount it using volumes.
 ### Example
 Example implementation for use with [amazon-ecr-credential-helper](https://github.com/awslabs/amazon-ecr-credential-helper):
 
-
 Use the dockerfile below to build the [amazon-ecr-credential-helper](https://github.com/awslabs/amazon-ecr-credential-helper),
 in a volume that may be mounted onto your watchtower container.
 
@@ -175,9 +173,12 @@ A few additional notes:
 
 1.  With docker-compose the volume (helper, in this case) MUST be set to `external: true`, otherwise docker-compose 
     will preface it with the directory name.
+
 2.  Note that "credsStore" : "ecr-login" is needed - and in theory if you have that you can remove the 
     credHelpers section
+
 3.  I have this running on an EC2 instance that has credentials assigned to it - so no keys are needed; however, 
     you may need to include the `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` environment variables as well.
+
 4.  An alternative to adding the various variables is to create a ~/.aws/config and ~/.aws/credentials files and 
     place the settings there, then mount the ~/.aws directory to / in the container.