docs: user-provided streamOrTableName for default write streams

[anguillanneuf]

Fixes a broken URL in a sample comment.

Also cleans up the docstrings for streamOrTableName in JsonStreamWriter.Builder.

IIUC, createDefaultStream can only be set to True on StreamWriter not JsonStreamWriter, the language below could mislead devs into thinking otherwise because it's not clear which builder it refers to.

java-bigquerystorage/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1beta2/JsonStreamWriter.java

Lines 185 to 188 in d6c3146

	* @param streamOrTableName name of the stream that must follow
	* "projects/[^/]+/datasets/[^/]+/tables/[^/]+/streams/[^/]+" or if it is default stream
	* (createDefaultStream is true on builder), then the name here should be a table name
	* ""projects/[^/]+/datasets/[^/]+/tables/[^/]+"

Also it looks like _default is not added by client code but by the library.

java-bigquerystorage/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1beta2/JsonStreamWriter.java

Lines 267 to 274 in d6c3146

	private Builder(String streamOrTableName, TableSchema tableSchema, BigQueryWriteClient client) {
	Matcher streamMatcher = streamPattern.matcher(streamOrTableName);
	if (!streamMatcher.matches()) {
	Matcher tableMatcher = tablePattern.matcher(streamOrTableName);
	if (!tableMatcher.matches()) {
	throw new IllegalArgumentException("Invalid name: " + streamOrTableName);
	} else {
	this.streamName = streamOrTableName + "/_default";

Not sure if I'm making all the right calls here. Thanks for reviewing!

---

Also see internal cl/381977832.

[stephaniewang526]

Thank you!