```Issue #13251 has been updated by Marcus Stollsteimer.

FYI, here my suggestion (will commit soon, need to read some HowTo's on how exactly that works, first):

``` patch
diff --git a/numeric.c b/numeric.c
index b34fb0a..2400a9c 100644
--- a/numeric.c
+++ b/numeric.c
@@ -5154,6 +5154,32 @@ DEFINE_INT_SQRT(BDIGIT, rb_bdigit_dbl, BDIGIT_DBL)

VALUE rb_big_isqrt(VALUE);

+/*
+ *  Document-method: Integer::sqrt
+ *  call-seq:
+ *     Integer.sqrt(n)  ->  integer
+ *
+ *  Returns the integer square root of the non-negative integer +n+,
+ *  i.e. the largest non-negative integer less than or equal to the
+ *  square root of +n+.
+ *
+ *    Integer.sqrt(0)        #=> 0
+ *    Integer.sqrt(1)        #=> 1
+ *    Integer.sqrt(24)       #=> 4
+ *    Integer.sqrt(25)       #=> 5
+ *    Integer.sqrt(10**400)  #=> 10**200
+ *
+ *  Equivalent to <code>Math.sqrt(n).floor</code>, except that the
+ *  result of the latter code may differ from the true value
+ *  due to the limited precision of floating point arithmetic.
+ *
+ *    Integer.sqrt(10**46)    #=> 100000000000000000000000
+ *    Math.sqrt(10**46).to_i  #=>  99999999999999991611392 (!)
+ *
+ *  If +n+ is not an Integer, it is converted to an Integer first.
+ *  If +n+ is negative, a Math::DomainError is raised.
+ */
+
static VALUE
rb_int_s_isqrt(VALUE self, VALUE num)
{
```

----------------------------------------
Bug #13251: [DOC] Add rdoc for Integer.sqrt
https://bugs.ruby-lang.org/issues/13251#change-63201

* Author: Marcus Stollsteimer
* Status: Assigned
* Priority: Normal
* Assignee: Marcus Stollsteimer
* Target version:
* ruby -v: ruby 2.5.0dev (2017-02-24 trunk 57707) [x86_64-linux]
* Backport: 2.2: DONTNEED, 2.3: DONTNEED, 2.4: DONTNEED
----------------------------------------
Nobu, I would offer to add documentation for Integer.sqrt, feature #13219 (r57705).

I'd like to prepare a patch and eventually try to commit it myself.

I'm not sure yet when to open issues for documentation and when to simply commit; I would be glad about any feedback.

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```