doc generation: do not insert line breaks (#1879)

The code generating markdown documentation from a Nickel configuration
used to unconditionally insert hard line breaks between a field's
signature and the rest of its documentation. However, one can't insert a
hard line break immediately followed by a blocks in commonmark (or put
differently: it's not representable as a source), and those line breaks
were just rendered as spurious backslashes at best, or sometimes
interpreted differently (see #1706 for more details).

A bit of experimentation showed that what comes after this line break is
always a block in the sense of commonmark (even when it's simple
documentation text, it's wrapped as a Markdown paragraph). Thus this
commit just removes the line break altogether, which is not useful and
never legal anyway.

cli/tests/snapshot/snapshots/snapshot__doc_stdout_record.ncl.snap

@@ -4,6 +4,4 @@ expression: out
 ---
 # `Hello`
 
-\
 World\!
-

cli/tests/snapshot/snapshots/snapshot__doc_stdout_types.ncl.snap

@@ -6,6 +6,4 @@ expression: out
 
 - `Hello : Number`
 - `Hello | std.string.Stringable`
-\
 World\!
-

core/src/program.rs

@@ -789,8 +789,6 @@ mod doc {
  ))
  }
 
- document.append(arena.alloc(AstNode::from(NodeValue::LineBreak)));
-
  if let Some(ref doc) = field.documentation {
  document.append(parse_markdown_string(header_level + 1, arena, doc, options));
  }