Update overview

doc/lambda2/overview.adoc

@@ -1,5 +1,5 @@
 ////
-Copyright 2020 Peter Dimov
+Copyright 2020, 2021 Peter Dimov
 Distributed under the Boost Software License, Version 1.0.
 https://www.boost.org/LICENSE_1_0.txt
 ////
@@ -12,16 +12,18 @@ https://www.boost.org/LICENSE_1_0.txt
 
 This is a simple, but functional, {cpp}14 lambda library. It takes
 advantage of the fact that the standard `<functional>` header already
-provides placeholders `_1`, `_2`, `_3`, and so on, for use with
-`std::bind`, and function objects such as `std::plus`, `std::greater`,
-`std::logical_not`, and `std::bit_xor`, corresponding to arithmetic,
-relational, logical and bitwise operators.
+provides `std::bind` customization points (`is_placeholder`,
+`is_bind_expression`), and function objects such as `std::plus`,
+`std::greater`, `std::logical_not`, and `std::bit_xor`, corresponding
+to arithmetic, relational, logical and bitwise operators.
 
 This allows the library to provide a minimal implementation that
 still lets expressions such as `_1 + 5`, `_1 % 2 == 0`, `_1 > _2`,
 or `_1 == ' ' || _1 == '\t'` to be composed and used as function
 objects.
 
+For example, `_1 + 5` is implemented as `std::bind(std::plus<>, _1, 5)`.
+
 These "lambda" expressions can also be freely combined with `std::bind`.
 For example, `std::bind( f, _1 ) == std::bind( g, _1 )` and
 `std::bind( f, _1 + _2 )` both work and have the expected behavior.
@@ -67,5 +69,5 @@ None. A single, self-contained header.
 * Clang 3.5 or later with `-std=c++14` or above
 * Visual Studio 2015, 2017, 2019
 
-Tested on https://travis-ci.org/github/pdimov/lambda2[Travis] and
+Tested on https://github.com/boostorg/lambda2/actions[Github Actions] and
 https://ci.appveyor.com/project/pdimov/lambda2[Appveyor].