`tns package-manager` docs does not list pnpm as supported value

[rosen-vladimirov]

Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project):

• CLI: 6.4.0
• Cross-platform modules: Not applicable
• Android Runtime: Not applicable
• iOS Runtime: Not applicable
• Plugin(s): Not applicable

Describe the bug
Help of tns package-manager get and tns package-manager set commands does not list the pnpm as supported value.

To Reproduce
tns package-manger set --help and tns package-manager get --help - no pnpm is described there.

Expected behavior
pnpm should be listed as supported value.