adding docs based on feedback

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/Nulls.kt

@@ -100,8 +100,17 @@ public fun <T, C> DataFrame<T>.fillNulls(vararg columns: ColumnReference<C>): Up
 
 // endregion
 
+/** Is only `true` if [this] is [Double.NaN] or [Float.NaN]. */
 internal inline val Any?.isNaN: Boolean get() = (this is Double && isNaN()) || (this is Float && isNaN())
 
+/**
+ * Returns `true` if [this] is considered NA.
+ * "NA", in DataFrame, roughly means `null` or `NaN`.
+ *
+ * Overload of `isNA` with contract support.
+ *
+ * @see NA
+ */
 @JvmName("isNaWithContract")
 @Suppress("NOTHING_TO_INLINE")
 @OptIn(ExperimentalContracts::class)
@@ -110,6 +119,11 @@ internal inline fun <T : Any?> T.isNA(): Boolean {
     return isNA
 }
 
+/**
+ * Is `true` if [this] is considered NA.
+ * "NA", in DataFrame, roughly means `null` or `NaN`.
+ * @see NA
+ */
 internal inline val Any?.isNA: Boolean
     get() = when (this) {
         null -> true
@@ -122,10 +136,22 @@ internal inline val Any?.isNA: Boolean
 
 internal inline val AnyCol.canHaveNaN: Boolean get() = typeClass.let { it == Double::class || it == Float::class }
 
+/**
+ * Is `true` when [this] column can have [NA] values.
+ * @see NA
+ */
 internal inline val AnyCol.canHaveNA: Boolean get() = hasNulls() || canHaveNaN || kind() != ColumnKind.Value
 
+/**
+ * Is `true` when [this] is `null` or [Double.NaN].
+ * @see NA
+ */
 internal inline val Double?.isNA: Boolean get() = this == null || this.isNaN()
 
+/**
+ * Is `true` when [this] is `null` or [Float.NaN].
+ * @see NA
+ */
 internal inline val Float?.isNA: Boolean get() = this == null || this.isNaN()
 
 // region fillNaNs

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/documentation/NA.kt

@@ -1,5 +1,7 @@
 package org.jetbrains.kotlinx.dataframe.documentation
 
+import org.jetbrains.kotlinx.dataframe.DataFrame
+import org.jetbrains.kotlinx.dataframe.DataRow
 import org.jetbrains.kotlinx.dataframe.api.dropNA
 import org.jetbrains.kotlinx.dataframe.api.fillNA
 
@@ -10,6 +12,10 @@ import org.jetbrains.kotlinx.dataframe.api.fillNA
  * [Floats][Float] or [Doubles][Double] can be represented as [Float.NaN] or [Double.NaN], respectively,
  * in cases where a mathematical operation is undefined, such as dividing by zero.
  *
+ * A [DataRow] can also be considered `NA` if each value inside is `NA`.
+ *
+ * A [DataFrame] is considered `NA` if it has no rows or columns, so if it's empty.
+ *
  * You can also use [fillNA][fillNA] to replace `NAs` in certain columns with a given value or expression
  * or [dropNA][dropNA] to drop rows with `NAs` in them.
  *

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/math/minMax.kt

@@ -58,7 +58,7 @@ private fun <T : Comparable<T>> Sequence<T>.bestOrNull(
         error(
             "Encountered non-comparable type ${
                 renderType(type)
-            } in $name function. Only self-comparable types are supported.",
+            } in $name function. Try converting the values to the same type `T : Comparable<T>`.",
         )
     }