docs: document the forked tree instead of switching the check off

[marevol]

#45 turned doclint's missing group off because the imported OpenSearch tree
carried 12,486 warnings. That was a workaround: it hid the finding instead of
fixing it, and it switched off the one javadoc group that says whether an API is
usable — for the whole repository, org.codelibs.fesen.client included.

The tree is documented now, and <doclint>all,-missing</doclint> is gone.

What was missing

count	finding
6,744	no comment
2,652	no @param
2,332	no @return
326	no @throws
216	no main description
212	use of default constructor, which does not provide a comment

Two passes were needed: a member with no comment at all is reported only as "no
comment", and its missing tags surface once it has one. The loop ran until javadoc
reported nothing.

How it was written

Every edit was placed at the file, line and column javadoc itself reported, so
existing comment text is never rewritten — tags are inserted into the block in
conventional order (@param, @return, @throws, then whatever the block already
had, including @opensearch.internal).

Prose is derived from the declaration, in the house style the client packages
already use:

/**
 * Returns the indices.
 *
 * @return the indices
 */
public Map<String, ClusterIndexHealth> getIndices() {

writeTo is "Writes this instance to the given output.", IOException is "if an
I/O error occurs", StreamInput constructors are "Creates a new X by reading it
from the given input.", a ParseField constant is "The NAME constant."

The 212 constructors

A default constructor has no source to attach a comment to, so nothing in a
comment can satisfy doclint. Each is now declared explicitly with the access the
compiler already gave it — the same access as its class, which is what an
implicit constructor has. Semantically a no-op; 205 public, 5 protected, 2
package-private.

Placement, four ways it goes wrong

These each broke a run before being handled:

• A doc comment under an annotation is not a doc comment. javac treats the
  annotation as the start of the declaration, so a /** */ after @Deprecated is
  an ordinary comment and the member stays undocumented. Comments go above the
  topmost annotation.
• no main description points inside the comment, not at the declaration —
  the opposite of every other finding. Anchoring on the reported line inserted a
  nested /** into an existing block.
• A trailing // note stops a line looking finished, so int x; // pkg-private
  read as a continuation and the next member's comment landed on it. Stripping it
  has to respect string literals and their escapes.
• Enum constants end in ,, so they all collapsed onto the first constant.

Verification

mvn javadoc:javadoc	BUILD SUCCESS — 0 errors, 0 warnings under -Xdoclint:all
mvn -Prelease javadoc:jar	BUILD SUCCESS — 0 errors, 0 warnings
mvn test	636 tests, 0 failures — OpenSearch 3/2/1 and Elasticsearch 8/7 containers included
generated pages	4,902 — unchanged

The diff is comments, blank lines and the 212 constructors. The only source lines
removed are single-line comments expanded to three, and one {} split open so a
constructor could go inside it.