Natspec parsing of custom tags always expects a value potentially swallowing other tags.

```
$ cat test.sol
contract C {
/// @custom:test
/// @param x test
function f(uint x) public pure {
}
}
$ build/solc/solc --devdoc test.sol
======= test.sol:C =======
Developer Documentation
{
"kind": "dev",
"methods":
{
"f(uint256)":
{
"custom:test": "@param x test"
}
},
"version": 1
}
```

Reproduce the behavior with test.sol using build/solc/solc --devdoc, focusing on the @custom:test and following @param x test tags. Trace the Natspec parsing entry point that produces the Developer Documentation output. Done means custom tags no longer consume the next tag as their value, while the documented tags remain parsed separately.

Written by the indexing model from the issue text.