Start docs for string type

docs/user/metrics/string.md

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+# String
+
+This allows recording a Unicode string value with arbitrary content.
+
+---
+
+_Note:_ This is not supporting recording JSON blobs - please get in contact with the Telemetry team if you're missing a type.
+
+---
+
+## Limits
+
+* Fixed maximum string length: 20 bytes. Specified in number of Unicode characters. Longer strings are truncated.
+
+## Examples
+
+* Record the operating system name with a value of "android".
+* Recording the device model with a value of "SAMSUNG-SGH-I997".
+
+## Recorded errors
+
+* `invalid_value`: if the string is too long