Base: Document readlink(1) and readlink(2) :^)

Author: https://github.com/bugaevc
Commit: https://github.com/SerenityOS/serenity/commit/fd985b1f48f
Pull-request: https://github.com/SerenityOS/serenity/pull/2564
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/awesomekling

Base/usr/share/man/man1/readlink.md

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+## Name
+
+readlink - get symlink target
+
+## Synopsis
+
+```**sh
+$ readlink [--no-newline] <path...>
+```
+
+## Description
+
+Print targets of the specified symbolic links.
+
+## Options
+
+* `-n`, `--no-newline`: Do not output a newline after each target
+
+## Examples
+
+```sh
+$ readlink /proc/self/cwd
+```
+
+## See also
+
+* [`readlink`(2)](../man2/readlink.md)

Base/usr/share/man/man2/readlink.md

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+## Name
+
+readlink - get symlink target
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+ssize_t readlink(const char* path, char* buffer, size_t size)
+```
+
+## Description
+
+`readlink()` writes up to `size` bytes of the target path of a symbolic link at
+the specified `path` to the given `buffer`. `readlink()` does not
+null-terminate the buffer. If the target of the link is longer than `size`
+bytes, it will get truncated.
+
+## Return value
+
+On success, `readlink()` returns the number of bytes written to the buffer,
+which is always less or equal to the specified `size`. Otherwise, it returns -1
+and sets `errno` to describe the error.
+
+## Notes
+
+The underlying system call always returns the full size of the target path on
+success, not the number of bytes written. `Core::File::read_link()` makes use
+of this to provide an alternative way to read links that doesn't require the
+caller to pick a buffer size and allocate a buffer straight up.
+
+Since it's essentially impossible to guess the right buffer size for reading
+links, it's strongly recommended that everything uses `Core::File::read_link()`
+instead.
+
+## Examples
+
+The following example demonstrates how one could implement an alternative
+version of `getpid(2)` which reads the calling process ID from ProcFS:
+
+```c++
+#include <LibCore/File.h>
+#include <unistd.h>
+
+pid_t read_pid_using_readlink()
+{
+    char buffer[64];
+    int rc = readlink("/proc/self", buffer, sizeof(buffer) - 1);
+    if (rc < 0)
+        return rc;
+    buffer[rc] = 0;
+    return atoi(buffer);
+}
+
+pid_t read_pid_using_core_file()
+{
+    auto target = Core::File::read_link("/proc/self");
+    if (target.is_null())
+        return -1;
+    auto pid = target.to_uint();
+    ASSERT(pid.has_value());
+    return pid.value();
+}
+```
+
+## See also
+
+* [`readlink`(1)](../man1/readlink.md)