Prevent the jsdoc filter from generating a comment closing its own block.

`/////...` was being converted to '/**//...' and causing a syntax
error. Now, any `*/` sequence in the resulting comment is converted to
`* /`. Also, only comment lines starting with exactly three slashes
are considered to be jsdoc comments. For ruby-style comments in YAML
files, jsdoc blocks must start with exactly two hash characters.

Closes #1874, #1884.