chore: rewrite user-facing messages in Support module

Conservative cleanup of frappe.throw/msgprint messages per the message style
guide; meaning, severity, and .format() arguments are unchanged:

- index bare {} placeholders as {0}/{1}/... so translators can reorder
- move f-strings / .format() / concatenation out of _() (they break gettext
  extraction and never translate)
- wrap translatable dynamic values (DocType/Select labels) in _()
- fix grammar and colloquialisms
- drop no-op _() wrapping runtime-built strings

Part of #53976.

erpnext/support/doctype/service_level_agreement/service_level_agreement.py

@@ -197,7 +197,7 @@ class ServiceLevelAgreement(Document):
 		)
 
 		if self.document_type not in valid_document_types:
-			frappe.throw(msg=_("Please select valid document type."), title=_("Invalid Document Type"))
+			frappe.throw(msg=_("Please select a valid document type."), title=_("Invalid Document Type"))
 
 	def validate_status_field(self):
 		meta = frappe.get_meta(self.document_type)