Improve `pmm-admin --help` descriptions for external services

I've a couple of issue while using adding external service:
1) if I specify --metrics-path=metrics it creates IP:9121metrics URL, but expected IP:9121/metrics.
So I think we should or improve our help and add an example how to specify metrics-path, or always add "/"

2) It's not clear how to use option "scheme"

  --scheme=SCHEME            Scheme to generate URI to exporter metrics endpoints

maybe we can add an example here too, at least "default: http" or something like that

Suggested implementation:
All changes will be in pmm-admin repo only.
1. Modify all metrics-path flags description that / will be included automatically if it doesnt contain already.

2. Add / before metrics-path value which doesnt contains / in beginning.

3. Modify all scheme flags description. To have more clear that it is actually protocol (http, https, etc).

Acceptance criteria:

• pmm-admin --help is more clear 
• Documentation Update
  1. pmm-admin reference manual is updated with new text 

How to test: 

1. Show help in pmm-admin, see if flag descriptions were modified.

2. Try add external service with flag metrics-path with and without / and then check URL for metrics if it contain / for both cases.