What is the SQL Server system table that contains information about stored procedure parameters?

You can query sys.procedures and sys.parameters...

select pr.name, p.*
from sys.procedures pr 
inner join sys.parameters p on pr.object_id = p.object_id

And join to types too...

select pr.name, p.*, t.name, t.max_length
from sys.procedures pr 
inner join sys.parameters p on pr.object_id = p.object_id
inner join sys.types t on p.system_type_id = t.system_type_id

You can also use

select * from INFORMATION_SCHEMA.PARAMETERS