Document: Add text arg when having blocks in say() calls

[seratch]

Summary

This pull request improves the code snippets in the "Getting Started" document by adding text argument when using blocks for composing a message. This enables the code to be more optimal in terms of usability plus compatible with TypeScript.

For context: #532 (comment)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #535 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #535   +/-   ##
=======================================
  Coverage   83.13%   83.13%           
=======================================
  Files           7        7           
  Lines         593      593           
  Branches      184      184           
=======================================
  Hits          493      493           
  Misses         68       68           
  Partials       32       32           

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[aoberoi]

great idea!