Fix markdown linting issues in WFL-io.md subprocess documentation

[coderabbitai]

Description

Five markdown linting issues need to be addressed in the subprocess execution documentation section of Docs/wfldocs/WFL-io.md:

1. Lines 555, 569, 678, 693 (MD036): Emphasis used instead of heading

   • Line 555: Change bold text to proper heading
   • Line 569: Change bold text to proper heading and use 'Allowlist' (single word)
   • Line 678: Change bold text to proper heading
   • Line 693: Change bold text to proper heading

2. Line 615 (MD040): Fenced code block missing language identifier

   • Add shell language identifier to the code fence

References

• Pull Request: Adds subprocess execution and management #185
• Review Comment: Adds subprocess execution and management #185 (comment)
• Requested by: @logbie

Note

This issue was created to address these linting concerns in a follow-up PR to keep the subprocess implementation PR focused on functionality.