Document SearchAttributes parsing

[lorensr]

Is your feature request related to a problem? Please describe.

SearchAttributes:

api/temporal/api/common/v1/message.proto

Lines 58 to 62 in 148f618

	// A user-defined set of *indexed* fields that are used/exposed when listing/searching workflows.
	// The payload is not serialized in a user-defined way.
	message SearchAttributes {
	map<string, Payload> indexed_fields = 1;
	}

I don't know from reading this proto how to specify search attribute types in StartWorkflowExecution. Possible types are:

api/temporal/api/enums/v1/common.proto

Lines 40 to 48 in df58bcd

	enum IndexedValueType {
	INDEXED_VALUE_TYPE_UNSPECIFIED = 0;
	INDEXED_VALUE_TYPE_TEXT = 1;
	INDEXED_VALUE_TYPE_KEYWORD = 2;
	INDEXED_VALUE_TYPE_INT = 3;
	INDEXED_VALUE_TYPE_DOUBLE = 4;
	INDEXED_VALUE_TYPE_BOOL = 5;
	INDEXED_VALUE_TYPE_DATETIME = 6;
	}

https://docs.temporal.io/docs/concepts/what-is-a-search-attribute/#custom-search-attributes

Describe the solution you'd like

A comment explaining how the Payloads are parsed, for example:

// Each Payload has a metadata.type: IndexedValueType
// If payload.metata.type is INDEXED_VALUE_TYPE_DATETIME, payload.data should be an integer with milliseconds since epoch.