[libc] Reorganize and clarify a few points around benchmarking

A few documentation clarifications and moving one part of the
docs around to be closer to the first mention of display so that
it's easier to spot based on some user feedback.

Differential Revision: https://reviews.llvm.org/D79443

Author: echristo

libc/utils/benchmarks/README.md

@@ -18,6 +18,7 @@ Then make sure to have `matplotlib`, `scipy` and `numpy` setup correctly:
 apt-get install python3-pip
 pip3 install matplotlib scipy numpy
 ```
+You may need `python3-gtk` or similar package for displaying benchmark results.
 
 To get good reproducibility it is important to make sure that the system runs in
 `performance` mode. This is achieved by running:
@@ -38,6 +39,26 @@ cmake -B/tmp/build -Sllvm -DLLVM_ENABLE_PROJECTS=libc -DCMAKE_BUILD_TYPE=Release
 make -C /tmp/build -j display-libc-memcpy-benchmark-small
 ```
 
+The display target will attempt to open a window on the machine where you're
+running the benchmark. If this may not work for you then you may want `render`
+or `run` instead as detailed below.
+
+## Benchmarking targets
+
+The benchmarking process occurs in two steps:
+
+1. Benchmark the functions and produce a `json` file
+2. Display (or renders) the `json` file
+
+Targets are of the form `<action>-libc-<function>-benchmark-<configuration>`
+
+ - `action` is one of :
+    - `run`, runs the benchmark and writes the `json` file
+    - `display`, displays the graph on screen
+    - `render`, renders the graph on disk as a `png` file
+ - `function` is one of : `memcpy`, `memcmp`, `memset`
+ - `configuration` is one of : `small`, `big`
+
 ## Benchmarking regimes
 
 Using a profiler to observe size distributions for calls into libc functions, it
@@ -62,22 +83,6 @@ Benchmarking configurations come in two flavors:
 _<sup>1</sup> - The size refers to the size of the buffers to compare and not
 the number of bytes until the first difference._
 
-## Benchmarking targets
-
-The benchmarking process occurs in two steps:
-
-1. Benchmark the functions and produce a `json` file
-2. Display (or renders) the `json` file
-
-Targets are of the form `<action>-libc-<function>-benchmark-<configuration>`
-
- - `action` is one of :
-    - `run`, runs the benchmark and writes the `json` file
-    - `display`, displays the graph on screen
-    - `render`, renders the graph on disk as a `png` file
- - `function` is one of : `memcpy`, `memcmp`, `memset`
- - `configuration` is one of : `small`, `big`
-
 ## Superposing curves
 
 It is possible to **merge** several `json` files into a single graph. This is