docs: add --field textarea multiline and underscore guidance (#61)

* docs: add --field textarea multiline and underscore guidance for atl CLI

- Document literal \n for multiline textarea fields
- Document expand section syntax (+++Title\ncontent\n+++)
- Add backtick guidance for code names with underscores
- Add --field markdown auto-conversion example

* fix: correct backtick escaping in atl textarea docs

Author: Hinne1

lib/llm/index.js

@@ -139,6 +139,7 @@ atl issue edit PROJ-1234 --add-label bug --remove-label wontfix
 atl issue edit PROJ-1234 --priority High
 atl issue edit PROJ-1234 --field "Story Points=8"        # Custom field by name
 atl issue edit PROJ-1234 --field "customfield_10016=8"    # Custom field by ID
+atl issue edit PROJ-1234 --field "Custom Field=Some **markdown** text"  # Auto-converts to ADF
 atl issue edit PROJ-1234 --field-file fields.json         # Complex values from JSON file
 
 # Workflow
@@ -335,6 +336,14 @@ Hidden content that can be expanded
 
 **Note**: Both descriptions and comments support full Markdown formatting (converted to ADF). Use \`--body-file <path>\` for multi-line or complex content to avoid shell escaping issues.
 
+**Textarea custom fields** (\`--field\`): Automatically converts Markdown to ADF. Use literal \`\\n\` for newlines:
+\`\`\`bash
+atl issue edit PROJ-1234 --field 'Kontext=Line 1\\n\\nLine 2\\n- Bullet A\\n- Bullet B'
+atl issue edit PROJ-1234 --field 'Kontext=+++Expand Title\\nHidden content\\n+++'
+\`\`\`
+
+**Code names with underscores**: Use backticks in descriptions (\`NegSoft_SN_Item\`). Bare underscores render as italic (\`NegSoft_SN_Item\` → NegSoft*SN*Item). Backslash-escaping (\`NegSoft\\_SN\\_Item\`) renders backslashes literally.
+
 ### Jira Workflow Transitions
 
 Transition names vary by Jira instance and language. Use \`atl issue transition PROJ-123 --list\` to see available transitions for a specific issue.