doc: spell out condition restrictions

Author: jkrems

doc/api/packages.md

@@ -689,6 +689,17 @@ exports, while resolving the existing `"node"`, `"node-addons"`, `"default"`,
 
 Any number of custom conditions can be set with repeat flags.
 
+Conditions can be almost any string, including multi-byte characters and
+whitespace. There's only a few restrictions:
+
+1. They must contain at least one character.
+1. They cannot start with "." since they may appear in places that also
+   allow relative paths.
+1. They cannot contain "," since they may be parsed as a comma-separated
+   list by some CLI tools.
+1. They cannot be numeric property keys like "10" since that can have
+   unexpected effects on property ordering.
+
 ### Community Conditions Definitions
 
 Condition strings other than the `"import"`, `"require"`, `"node"`,