Introduce documentation for PCLA implementation

^KT-65637 Fixed

docs/fir/builder_inference.md

@@ -1,6 +1,8 @@
 ## FIR/Builder inference
 See also: [Kotlin Spec: Builder-style type inference](https://kotlinlang.org/spec/type-inference.html#builder-style-type-inference)
 
+**NB:** This document is quite obsolete, see [pcla.md](pcla.md).
+
 ### Glossary
 #### Postponed type variable
 Type-variable, that was used to build a stub type on, and fixation of which is postponed until the end of lambda analysis.
@@ -217,4 +219,4 @@ It leads to violation of contract in `org.jetbrains.kotlin.fir.resolve.inference
 To avoid such problem, we have [workaround for lambdas with Unit return-type](#Note-Incomplete-calls-in-return-arguments), but problem still 
 occurs:
 - When we have [lambdas with non-Unit return-type](https://youtrack.jetbrains.com/issue/KT-58741)
-- When the [incomplete call is present in a return argument that isn't last expression](https://youtrack.jetbrains.com/issue/KT-58742)
+- When the [incomplete call is present in a return argument that isn't last expression](https://youtrack.jetbrains.com/issue/KT-58742)