Document required quotes in Mermaid subgraph titles

Mermaid v8.2.3 added support for styling in subgraphs, but if there are multiple words in a title, quotes are required.

Works

graph TB

  SubGraph1 --> SubGraph1Flow
  subgraph "SubGraph 1 Flow"
  SubGraph1Flow(SubNode 1)
  SubGraph1Flow -- Choice1 --> DoChoice1
  SubGraph1Flow -- Choice2 --> DoChoice2
  end

  subgraph "Main Graph"
  Node1[Node 1] --> Node2[Node 2]
  Node2 --> SubGraph1[Jump to SubGraph1]
  SubGraph1 --> FinalThing[Final Thing]
end

Does NOT work

graph TB

  SubGraph1 --> SubGraph1Flow
  subgraph SubGraph 1 Flow
  SubGraph1Flow(SubNode 1)
  SubGraph1Flow -- Choice1 --> DoChoice1
  SubGraph1Flow -- Choice2 --> DoChoice2
  end

  subgraph Main Graph
  Node1[Node 1] --> Node2[Node 2]
  Node2 --> SubGraph1[Jump to SubGraph1]
  SubGraph1 --> FinalThing[Final Thing]
end

• https://github.com/knsv/mermaid/pull/845
• https://github.com/knsv/mermaid/issues/391#issuecomment-495602059

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65338